Two approaches for net ecosystem carbon budgets and soil carbon sequestration in a rice–wheat rotation system in China

Few studies have comprehensively evaluated the method of estimating the net ecosystem carbon budget (NECB). We compared two approaches for studying the NECB components on the crop seasonal scale as validated by the soil organic carbon (SOC) changes measured over the 5-year period of 2009–2014. The field trial was initiated with four integrated soil–crop system management (ISSM) practices at different nitrogen application rates relative to the local farmer’s practices (FP) rate, namely, N1 (25 % reduction), N2 (10 % reduction), N3 (FP rate) and N4 (25 % increase) with no nitrogen (NN) and FP as the controls. Compared with the FP, the four ISSM scenarios of N1, N2, N3 and N4 significantly increased rice yields by 9.5, 19, 33 and 41 %, while increasing the agronomic nitrogen use efficiency (NUE) by 71, 75, 99 and 79 %, respectively. The SOC sequestration potentials were estimated to be ?0.15 to 0.35 Mg C ha^?1 year^?1 from the net primary production minus heterotrophic respiration approach and ?0.32 to 0.67 Mg C ha^?1 year^?1 from the gross primary production minus ecosystem respiration approach for the 2010–2011 rice–wheat annual cycle. Similarly, the annual topsoil carbon sequestration rate over 2009–2014 was measured to be ?0.22 Mg C ha^?1 year^?1 for the NN plot and 0.13–0.42 Mg C ha^?1 year^?1 for the five fertilized treatments. Both NECB approaches provided a sound basis for accurate assessment of the SOC changes. Compared to the SOC sequestration rate from the FP, the proposed N3 and N4 scenarios increased the SOC sequestration rates while also improving rice yield and NUE.     

Differential retention of carbon, nitrogen and phosphorus in grassland soil profiles with long-term manure application

Liquid hog manure (LHM) is a valuable source of nutrients for farm production. Long-term experimental plots that had received LHM applications of 0, 50, and 100 m³ ha^?1 annually for 20 years were analyzed for total soil C, N and P storage. Applications increased total soil N and P by 1,200 kg N ha^?1 and 850 kg P ha^?1 at 100 m^?3 LHM year^?1, compared to the control treatment. However, C storage did not increase with LHM rates and was lower in the 50 m³ ha^?1 LHM treatment (86 Mg C ha^?1) than in the 0 or 100 m³ ha^?1 treatments (100 Mg C ha^?1). In addition to the limited quantities and high decomposability of the C supplied by LHM, it is hypothesized that LHM stimulated the mineralization of both native soil C and fresh root-derived material. This priming effect was particularly apparent in deeper soil horizons where the decomposability of native C may be limited by the supply of fresh C. This study indicates that while LHM can be a significant source of crop nutrients, it has limited capacity for maintaining or increasing soil C.     

Can litter production and litter decomposition improve soil properties in the rubber plantations of different ages in Côte d’Ivoire?

Litter production and litter decomposition influence the availability of nutrients in the soil. The investigation aimed at characterizing the dynamics of leaf litter decomposition, and soil physico-chemical and biological parameters in rubber plantations of different ages. During a 12-months’ period, field studies were done in 7-, 12-, and 25-year-old rubber plantations. For measuring of litter decomposition and input from aboveground, 324 litter bags and 27 litter traps (1 m?×?1 m) were placed in 3 sampling areas per age class of rubber plantations. The soil parameters were also characterized. The results showed that the annual litter production and the amounts of organic carbon in leaves increased with the aging of the plantations. The annual decomposition constant (k) ranged from 0.0381?±?0.0040 year^?1 in the 25-year-old plantations to 0.0767?±?0.0111 year^?1 in the 7-year-old plantations. The annually decomposed litter mass varied between 2.7?±?0.3 t ha^?1 year^?1 in the 12-year-old plantations to 4.2?±?0.3 t ha^?1 year^?1 in the 25-year-old plantations. The soil of the 25-year-old plantations showed higher values of most physico-chemical and biological variables as compared to the 7-year-old plantations: annual litter production (+?32%), annual litter mass decomposed (+?11%), annual carbon (+?15%) and nitrogen (+?11%) inputs, soil organic carbon (+?52%), total nitrogen (+?32%), soil organic matter (+?52%), soil water content (+?74%), and the total density of soil invertebrates (+?121%). The results indicate an improvement of soil properties with the aging of the rubber plantations and the importance of this agricultural system for carbon sequestration.     

Net ecosystem CO2 exchange and carbon cycling in tropical lowland flooded rice ecosystem

The seasonal fluxes of CO₂ and its characteristics with relation to environmental variables were investigated under tropical lowland flooded rice paddies employing the open path eddy covariance technique. The seasonal net ecosystem carbon budget was quantified by empirical modelling approach. The integrated net ecosystem exchange (NEE), gross primary production (GPP) and ecosystem respiration (RE) in the flooded rice field was ?448, 811 and 363 g C m^?2 in wet season. Diurnal variations of mean NEE values during the season varied from +3.99 to ?18.50 μmol CO₂ m^?2 s^?1. The daily average NEE over the cropping season varied from +2.73 to ?7.74 g C m^?2 day^?1. The net ecosystem CO₂ exchange reached its maximum in heading to flowering stage of rice with an average value of ?5.67 g C m^?2 day^?1. On daily basis the flooded rice field acted as a net sink for CO₂ during most of the times in growing season except few days at maturity when it became a net CO₂ source. The rate of CO₂ uptake by rice as observed from negative NEE values increased proportionally with air temperature up to 34 °C. The carbon distribution in different component of soil-plant system namely, soil organic carbon, dissolved organic carbon, methane emission, rhizodeposition, carbon in algal biomass, crop harvest and residues were quantified and carbon balance sheet was prepared for the wet season in tropical rice. Carbon balance sheet for tropical rice revealed 7.12 Mg C ha^?1 was cycled in the system in wet season.     

Carbon and nutrient fluxes and balances in Nuba Mountains homegardens,Sudan

Management intensification has raised concerns about the sustainability of homegardens in the Nuba Mountains, Sudan. This study aimed at assessing the sustainability of these agroecosystems following the approach of carbon (C) and nutrient balances. Three traditional (low input) and three intensified (high input) homegardens were selected for monitoring of relevant input and output fluxes of C, nitrogen (N), phosphorus (P) and potassium (K). The fluxes comprised those related to management activities (soil amendments, irrigation, and biomass removal) as well as estimates of biological N₂ fixation, C fixation by photosynthesis, wet and dry deposition, gaseous emission, and leaching. Annual balances for C and nutrients amounted to ?21 kg C ha^?1, ?70 kg N ha^?1, 9 kg P ha^?1 and ?117 kg K ha^?1 in high input homegardens and to ?1,722 kg C ha^?1, ?167 kg N ha^?1, ?9 kg P ha^?1 and ?74 kg K ha^?1 in low input homegardens. Photosynthesis C was the main C input flux with averaged of 7,047 and 5,610 kg C ha^?1 a^?1 in high and low input systems, respectively. Biological N₂ fixation (17 kg N ha^?1 a^?1) was relevant only in low input systems. In both systems, the annual input of 77 kg K ha^?1 through dust was highly significant and annual gaseous C losses of about 5,900 kg C ha^?1 were the main C loss. In both garden types, the removal of biomass accounted for more than half of total nutrient exports of which one-third resulted from weeding and removal of plant residues and two-third from harvest. The observed negative nutrient balances may lead to a long-term decline of crop yields. Among other measures the reuse of C and nutrients in biomass removals during the cleaning of homegardens may allow to partially close C and nutrient cycles.     

Nitrogen balance in dryland agroecosystem in response to tillage,crop rotation,and cultural practice

Accounting of N inputs and outputs and N retention in the soil provides N balance that measures agroecosystem performance and environmental sustainability. Because of the complexity of measurements of some N inputs and outputs, studies on N balance in long-term experiments are scanty. We examined the effect of 8 years of tillage, crop rotation, and cultural practice on N balance based on N inputs and outputs and soil N sequestration rate under dryland cropping systems in the northern Great Plains, USA. Tillage systems were no-tillage (NT) and conventional tillage (CT) and crop rotations were continuous spring wheat (Triticum aestivum L.) (CW), spring wheat–pea (Pisum sativum L.) (W–P), spring wheat–barley (Hordeum vulgaris L.) hay–pea (W–B–P), and spring wheat–barley hay–corn (Zea mays L.)–pea (W–B–C–P). Cultural practices were traditional (conventional seed rates and plant spacing, conventional planting date, broadcast N fertilization, and reduced stubble height) and improved (variable seed rates and plant spacing, delayed planting, banded N fertilization, and increased stubble height). Total N input due to N fertilization, pea N fixation, atmospheric N deposition, crop seed N, and nonsymbiotic N fixation was greater with W–B–C–P than CW, regardless of tillage and cultural practices. Total N output due to aboveground biomass N removal and N losses due to denitrification, volatilization, plant senescence, N leaching, gaseous N (NO_x) emissions, and surface runoff were not different among treatments. Nitrogen sequestration rate at 0–20 cm from 2004 to 2011 varied from 29 kg N ha^?1 year^?1 in CT with W–P to 89 kg N ha^?1 year^?1 in NT with W–P. Nitrogen balance varied from ? 39 kg N ha^?1 year^?1 in NT with CW and the improved practice to 41 kg N ha^?1 year^?1 in CT with W–P and the traditional practice. Because of legume N fixation and increased soil N sequestration rate, diversified crop rotations reduced external N inputs and increased aboveground biomass N removal, N flow, and N balance compared with monocropping, especially in the CT system. As a result, diversified legume–nonlegume crop rotation not only reduced the cost of N fertilization by reducing N fertilization rate, but also can be productive by increasing N uptake and N surplus and environmentally sustainable by reducing N losses compared with nonlegume monocropping, regardless of cultural practices in dryland agroecosystems.     

Corn and soybean’s season-long in-situ nitrogen mineralization in drained and undrained soils

Many factors influence nitrogen (N) mineralization in agricultural soils. Our objective was to quantify cumulative (season-long) net N mineralization in corn (Zea mays L.) and soybean [Glycine max (L.) Merr] in a corn-soybean rotation under different N and soil drainage management (drained and undrained) in poorly-drained soils. In-situ incubations were conducted over two growing seasons using a sequential core-sampling technique to measure net N mineralization. Differential drainage was imposed three-years before this study, in which time, the soil lost 2.2 Mg C ha^?1 year^?1 and 0.14 Mg N ha^?1 year^?1 due to tile-drainage. Overall greater total soil organic carbon (TOC) and total soil nitrogen (TN) in the undrained soil resulted in 2.7 times greater net N mineralization compared to the drained soil in the unfertilized control (0N), but the effect of drainage was inconsistent across years with N fertilization. Across all variables, soils mineralized 2.89% of TN in soybean residue and 0.94% of TN in corn residue. Nitrogen fertilization increased mineralization rate, as high as 9.6 kg N ha^?1 day^?1, compared to <2.2 kg N ha^?1 day^?1 for 0N. Overall, net N mineralization was 3.4 times greater with N fertilizer than the 0N, but fertilization made mineralization more variable. The impact of fertilization on boosting mineralization under differential soil drainage needs further refinement if we are to improve decision-making tools for N application based on soil mineralization predictions.     

Crop yields and soil organic carbon fractions as influenced by straw incorporation in a rice–wheat cropping system in southeastern China

In agro-ecosystems, the relationship between soil fertility and crop yield is mediated by manure application. In this study, an 8-year field experiment was performed with four fertilizer treatments (NPK, NPKM₁, NPKM₂, and NPKM₃), where NPK refers to chemical fertilizer and M₁, M₂, and M₃ refer to manure application rates of 15, 30, and 45 Mg ha^?1 year^?1, respectively. The results showed that the NPKM (NPKM₁, NPKM₂, and NPKM₃) treatments produced greater and more stable yields (4.95–5.45 Mg ha^?1 and 0.59–0.75) than the NPK treatment (4.01 Mg ha^?1 and 0.50). Crop yields under the NPKM treatments showed two trends, with a rate of decrease of 0.48–0.83 Mg ha^?1 year^?1 during the first 4 years and a rate of increase of 0.10–0.25 Mg ha^?1 year^?1 during the last 4 years. The soil organic carbon (SOC) significantly increased under all treatments. The estimated annual SOC decomposition rate was 0.35 Mg ha^?1 year^?1 and the equilibrium SOC level was 6.22 Mg ha^?1. Soil total nitrogen (N), available N, total phosphorus (P) and available P under the NPKM treatments increased by 0.15–0.26, 15–33, 0.17–0.66 and 45–159 g kg^?1, respectively, compared with the NPK treatment. Manure application mainly influenced crop yield by affecting the soil TN, available N, and available P, which accounted for up to 64% of the crop yield variation. Taken together, applying manure can determine or at least improve the effects of soil fertility on crop yield in acidic soils in South China.     

Quantification of nitrogen inputs from biological nitrogen fixation to whole farm nitrogen budgets of two dairy farms in Atlantic Canada

Legume biological N fixation (BNF) is a large source of uncertainty in farm N budgets. This study sought to quantify the BNF-N input to two whole farm nitrogen budgets and establish a simple and accurate method for incorporating BNF values as inputs in whole farm N budgets. Nitrogen inputs and outputs as well as flows of N between animal and crop production components were determined for a dairy farm in New Brunswick (NB) and Prince Edward Island (PE) over a two year period. The ¹⁵N natural abundance method was used to determine the %N derived from the atmosphere (%Ndfa) through BNF at both sites. Red clover (Trifolium pratense) at the PE site derived 77 % of its N from BNF and alfalfa (Medicago sativa) collected at both the PE and NB farms derived 72 % of its N from BNF. Total BNF-N present in legume biomass from mixed forage fields measured with the ¹⁵N natural abundance method ranged from 39 to 116 kg N ha^?1 year^?1. A legume dry matter conversion model adjusted with %Ndfa and %N of red clover and alfalfa samples from both farm sites was selected to estimate BNF-N inputs from mixed forage fields on the farms. Averaged across the entire cropland area at each farm site, the BNF-N inputs ranged from 27 to 52 kg N ha^?1 year^?1. The farmgate BNF-N inputs are low in comparison to other studies, possibly due to low legume contents in forage fields. BNF accounted for 18–29 % of farmgate N inputs at the farms. Surpluses of N found at both farm sites ranged from 98 to 135 kg N ha^?1 year^?1, typical to the whole farm N budgets of similar dairy farms.     

Seasonal N<Subscript>2</Subscript>O emissions respond differently to environmental and microbial factors after fertilization in wheat–maize agroecosystem

Biogeochemical processes regulating cropland soil nitrous oxide (N₂O) emissions are complex, and the controlling factors need to be better understood, especially for seasonal variation after fertilization. Seasonal patterns of N₂O emissions and abundances of archaeal ammonia monooxygenase (amoA), bacterial amoA, nitrate reductase (narG), nitrite reductase (nirS/nirK), and nitrous oxide reductase (nosZ) genes in long-term fertilized wheat–maize soils have been studied to understand the roles of microbes in N₂O emissions. The results showed that fertilization greatly stimulated N₂O emission with higher values in pig manure-treated soil (OM, 2.88 kg N ha^?1 year^?1) than in straw-returned (CRNPK, 0.79 kg N ha^?1 year^?1) and mineral fertilizer-treated (NPK, 0.90 kg N ha^?1 year^?1) soils. Most (52.2–88.9%) cumulative N₂O emissions occurred within 3 weeks after fertilization. Meanwhile, N₂O emissions within 3 weeks after fertilization showed a positive correlation with narG gene copy number and a negative correlation with soil NO₃^? contents. The abundances of narG and nosZ genes had larger direct effects (1.06) than ammonium oxidizers (0.42) on N₂O emissions according to partial least squares path modeling. Stepwise multiple regression also showed that log narG was a predictor variable for N₂O emissions. This study suggested that denitrification was the major process responsible for N₂O emissions within 3 weeks after fertilization. During the remaining period of crop growth, insufficient N substrate and low temperature became the primary limiting factors for N₂O emission according to the results of the regression models.     

Tree-scale spatial variation of soil respiration and its influence factors in apple orchard in Loess Plateau

Although small-scale spatial variation of soil respiration has been studied in a wide variety of ecosystems, there are few studies investigating the spatial variation of soil respiration at tree-scale. An inaccurate estimation of soil respiration would be obtained if the spatial variation of soil respiration was ignored. Soil respiration, soil temperature, soil moisture and fine roots biomass were measured in different directions (0, 120, and 240°) at different distances (0.5 and 2 m radial distance) from the trunk of three representative trees for the period 2011–2013 in a mature apple orchard established on the Loess Plateau in 2000. The mean soil respiration rate at 0.5 m-distance was 21, 35 and 42 % higher, respectively. The cumulative soil respiration at 0.5 m-distance was 20, 31, and 38 % higher; and the temperature sensitivity of soil respiration (Q ₁₀) at 0.5 m-distance was 15, 30 and 12 % higher than that at 2 m-distance in 2011, 2012, and 2013, respectively. There was no significant difference in soil temperature and moisture between 0.5 m- and 2 m-distance, whereas fine root biomass at 0.5 m-distance was 64, 108, and 114 % higher than that at 2 m-distance in 2011–2013, respectively. Fine root biomass had a positive linear relationship with accumulative soil respiration and Q ₁₀. Mean annual cumulative soil respiration was 0.46, 0.45, and 0.57 kg C m^?2 year^?1 in 2011–2013, respectively. Fine root biomass contributed to the spatial variation of soil respiration in apple orchard, and soil respiration at 2 m-distance could represent the C respired in orchard level.     

Symbiotic nitrogen fixation by legumes in two Chinese grasslands estimated with the 15N dilution technique

Symbiotic nitrogen (N) fixation by legumes was investigated using the ¹⁵N dilution technique in two Chinese grasslands: one in the north-eastern Tibetan Plateau and the other in Inner Mongolia in China. A small amount (0.03 g N m^?2) of ¹⁵N labelled (NH₄)₂SO₄ fertilizer was evenly distributed in two soils. One month after the ¹⁵N addition, four legumes (Astragalus sp., Gueldenstaedtia diversifolia, Oxytropis ochrocephala and Trigonella ruthenica) in the alpine meadow and two legumes (Thermopsis lanceolata and Melissitus ruthenica) in the temperate steppe were collected. Several non-legume plant species were harvested as the reference. Above-ground biomass of legumes ranged from 8 to 24 g m^?2 in the alpine meadow and from 11 to 35 g m^?2 in the temperate steppe. The reference plants showed distinctly higher ¹⁵N atom% excess than legumes (0.08% vs. 0.02% in the alpine meadow, 0.10% vs. 0.02% in the temperate steppe). The N derived from atmosphere (%Ndfa) ranged from 50 to 90% N in the alpine meadow, while it ranged from 85 to 92% in the temperate steppe. Based on the legume above-ground biomass, total symbiotic N₂-fixation rate was estimated to be 1.00 g N m^?2 year^?1 in the alpine meadow and 1.15 g N m^?2 year^?1 in the temperate steppe. These N inputs by legumes can account for 9% of the gap between the N demand and the seasonal N release by mineralization in the alpine Kobresia grassland and 20% in the temperate Leymus grassland, respectively. Considering additional contribution of the root biomass, we suggest that biological N₂-fixation by legumes plays an important role in the cycling of N in both Kobresia and Leymus grasslands on an annual scale.     

Solid Phase Extraction and Determination of Ultra Trace Amounts of Copper using Activated Carbon Modified by N,N′‐Bis(Salicylidene)‐1,2‐Phenylenediamine

Abstract

N,N′‐bis(salicylidene)‐1,2‐phenylenediamine(salophen) modified activated carbon was prepared and used as an effective sorbent for solid phase extraction of Cu(II) ions from aqueous solutions. The salophen modified activated carbon showed a high sorption affinity for Cu(II). In this method a column mode was used for preconcentration of copper(II) in the pH range 3.5–6.5. The retained copper was eluted with 0.1 mol l^?1 EDTA and determined by atomic absorption spectrometry. The calibration graph was linear over the copper concentration in the range 0.05–1.5 µg ml ^?1. Five replicate determination of 0.4 µg ml^?1 of copper(II) gave a mean absorbance of 0.385 with a relative standard deviation of 1.35%. The detection limit was 0.0133 µg ml^?1. The interference of a large number of anions and cations has been studied and the optimized conditions developed were utilized for determination of copper in various real samples.     

Effects of pig and dairy slurry application on N and P leaching from crop rotations with spring cereals and forage leys

Two crop rotations dominated by spring cereals and grass/clover leys on a clay soil were studied over 2 years with respect to nitrogen (N) and phosphorus (P) leaching associated with pig or dairy slurry application in April, June and October. Leaching losses of total N (TN), total P (TP), nitrate-N and dissolved reactive P (DRP) were determined in separately tile-drained field plots (four replicates). Mean annual DRP leaching after October application of dairy slurry (17 kg P ha^?1) to growing grass/clover was 0.37 kg ha^?1. It was significantly higher than after October application of pig slurry (13 kg ha^?1) following spring cereals (0.16 kg ha^?1) and than in the unfertilised control (0.07 kg P ha^?1). The proportion of DRP in TP in drainage water from the grass/clover crop rotation (35 %) was higher than from the spring cereal rotation (25 %) and the control (14 %). The grass/clover rotation proved to be very robust with respect to N leaching, with mean TN leaching of 10.5 kg ha^?1 year^?1 compared with 19.2 kg ha^?1 year^?1 from the cereal crop rotation. Pig slurry application after cereals in October resulted in TN leaching of 25.7 kg ha^?1 compared with 7.0 kg ha^?1 year^?1 after application to grass/clover in October and 19.1 kg ha^?1 year^?1 after application to spring cereals in April. In conclusion, these results show that crop rotations dominated by forage leys need special attention with respect to DRP leaching and that slurry application should be avoided during wet conditions or combined with methods to increase adsorption of P to soil particles.     

Growth,grain yield and nitrogen use efficiency of Mediterranean wheat in soils amended with municipal sewage sludge

The application of sewage sludge (SS) to agricultural land can improve soil fertility and physical properties, and enhance crop production. This field study was conducted for two consecutive growing seasons to investigate the influence of SS application on winter wheat growth, grain yield, N accumulation, translocation and use, and on trace elements concentrations in soil and wheat plants under Mediterranean conditions. Treatments consisted of three rates of SS, i.e. 20, 40, and 60 Mg dry weight ha^?1 year^?1, one rate of inorganic fertilizer (IF, 120 kg N ha^?1 year^?1 plus 80 kg P₂O₅ ha^?1 year^?1), and an unamended control. The application of SS resulted in tall plants with high early dry matter and N accumulation similar to or significantly higher than those obtained with IF. The lowest SS application rate resulted in grain yield similar to that obtained with IF. Nitrogen use efficiency (NUE) in SS treatments was mainly determined by uptake efficiency, which decreased with increasing SS application rate. Values of NUE and biomass production efficiency with the lowest SS rate were similar to those obtained with IF. SS application resulted in increased concentrations of total and DTPA-extractable trace elements in the soil after the first year, but concentrations were much lower than the regulation limits. Concentrations of Cu, Mn and Zn in wheat plants did not exceed those obtained with IF. Overall, SS could be considered for use as a fertilizer in wheat production systems in the area, serving also as an alternative method of SS disposal.     

Soil organic matter,greenhouse gases and net global warming potential of irrigated conventional,reduced-tillage and organic cropping systems

Reducing tillage intensity and diversifying crop rotations may improve the sustainability of irrigated cropping systems in semi-arid regions. The objective of this study was to compare the greenhouse gas (GHG) emissions, soil organic matter, and net global warming potential (net GWP) of a sugar beet (Beta vulgaris L.)-corn (Zea mays L,) rotation under conventional (CT) and reduced-tillage (RT) and a corn-dry bean (Phaseolus vulgaris L.) rotation under organic (OR) management during the third and fourth years of 4-year crop rotations. The gas and soil samples were collected during April 2011–March 2013, and were analyzed for carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions, water-filled pore space (WFPS), soil nitrate (NO₃ ^?–N) and ammonium (NH₄ ⁺–N) concentrations, soil organic carbon (SOC) and total nitrogen (TN), and net global warming potential (net GWP). Soils under RT had 26% lower CO₂ emissions compared to 10.2 kg C ha^?1 day^?1 and 43% lower N₂O emissions compared to 17.5 g N ha^?1 day^?1 in CT during cropping season 2011, and no difference in CO₂ and N₂O emissions during cropping season 2012. The OR emitted 31% less N₂O, but 74% more CO₂ than CT during crop season 2011. The RT had 34% higher SOC content than CT (17.9 Mg ha^?1) while OR was comparable with CT. Net GWP was negative for RT and OR and positive for CT. The RT and OR can increase SOC sequestration, mitigate GWP and thereby support in the development of sustainable cropping systems in semiarid agroecosystems.     

Comparison of net ecosystem CO2 exchange in cropland and grassland with an automated closed chamber system

Field measurements of net ecosystem CO₂ exchange (NEE) with high temporal resolution are essential to construct a meaningful ecosystem C balance. The objectives of this study were to monitor NEE in high temporal resolution in cropland and grassland between middle August and middle November (2006) at Kleinhohenheim, Germany and to evaluate NEE in autumn. A fully automated temperature controlled closed chamber system with an infrared CO₂ analyzer was used to measure NEE. The measured NEE varied between the two ecosystems depending on changes in above-ground vegetation and environmental factors. The diurnal NEE pattern of daytime CO₂ uptake and night time CO₂ release was evident in the grassland, but not in the cropland as the crops were harvested at the beginning of the measurement period. The grassland generally showed higher night time NEE, but lower daytime NEE than the cropland. Night time NEE showed exponential dependence on air and soil temperature, resulting in Q₁₀ of 1.8 and 1.9 (for air temperature), 2.3 and 2.4 (for soil temperature) in the grassland and cropland, respectively. The average daily NEE was 2.77 and 1.86 g CO₂-C m^?2 day^?1 in the cropland and grassland, respectively. Both ecosystems were sources of CO₂, during 3 months in autumn, but the grassland emitted less CO₂ by 87.9 g CO₂-C m^?2 than the cropland.     

Assessment of nutrient returns in a tropical dry forest after clear-cut without burning

Tropical dry forests (TDFs) are being deforested at unprecedented rates. The slash/burn/agriculture/fallow-extensive livestock sequence causes significant nutrient losses and soil degradation. Our aim is to assess nutrient inputs and outputs in a TDF area under an alternative management system, for exclusive wood production. The study involved clear-cutting a preserved caatinga TDF site without burning, quantifying nutrients exported in firewood/timber and nutrients returned to the soil from the litter layer plus the slash debris, left to decompose unburned on the soil surface. Before clear-cut, the litter layer on the forest floor contained 6.1 t ha of dry matter (DM). After clear-cut, the aboveground biomass was 61.9 t DM ha^?1 (consisting of 21.5 t DM ha^?1 of commercial wood and 40.4 t DM ha^?1 of clear-cut debris that did not include the underlying litter layer). The litter layer was composed of fine and coarse litter, with turnovers of 0.86 and 0.31 year^?1, respectively, separately measured in uncut control plots during two rainy seasons (Dec-2007/June-2008 and Dec-2008/June-2009). In a single season, its decomposition returned to the soil 48.4, 1.16 and 12.3 kg ha^?1 of N, P and K. The clear-cut debris was mainly composed of branches, 33.4 t ha^?1, bromeliads, 5.63 t ha^?1 and green leaves, 1.32 t ha^?1. In-situ decomposition rates for branches and bromeliads were 0.24 and 1.47 year^?1, respectively. After two rainy seasons the clear-cut debris released 206, 6.5 and 106 kg ha^?1 of N, P and K respectively. This input plus that of the underlying litter layer exceeded exports in the commercial wood, and replenished a soil nutrient stock (0–30 cm) of approximately the same magnitude.     

Stream water nutrient and organic carbon exports from tropical headwater catchments at a soil degradation gradient

Carbon and nutrient losses were quantified from four small headwater catchments in western Kenya in the year 2008. They include a forested catchment and three catchments under maize continuously cultivated for 5, 10 and 50 years following forest conversion. The C isotopic composition of dissolved organic C (DOC) in stream discharge suggested that soil organic C (SOC) derived from the original forest rather than OC from maize may have contributed to a large extent to watershed OC losses, even 50 years after the forest was removed. Flow-weighted stream water concentrations of DOC and coarse particulate OC, all N species, total P, K and Na significantly (P < 0.05) increased in streams after forest conversion and long-term cultivation. Solute concentrations increased despite the fact that soil contents decreased and total water flow increased indicating mobilization of C and N, P and K from soil with progressing cultivation. In contrast, Ca and Mg concentrations in stream water did not systematically change after deforestation and cultivation, and may be controlled by geochemical weathering rather than by changing water flow paths or topsoil contents. All OC and nutrient exports increased with longer cultivation over decadal time scales (P < 0.05) to the same or greater extent than through deforestation and the first years of cultivation. Fluvial OC and total N losses were 2 and 21 % of total SOC and total N decline, respectively, in the top 0.1 m over 50 years. Fluvial OC losses therefore played a minor role, and SOC losses were mainly a result of microbial mineralization. Resulting total N losses by stream discharge, however, were large with 31 kg ha^?1 year^?1 after 50 years of continuous cropping in comparison to fertilization of 40 kg N ha^?1 year^?1. Most (91 %) of the N losses occurred as NO₃ ^?. In contrast, P losses by stream discharge were negligible in comparison to plant uptake. Water losses should be managed to reduce soil fertility declines especially through large N export from agricultural headwater catchments. However, stream concentrations of both P (0.01–0.15 mg L^?1) and N (0.4–4.8 mg L^?1) were moderate or low with respect to possible consequence for human health and not responsible for eutrophication observed in Lake Victoria.     

Silicon cycle in rice paddy fields: insights provided by relations between silicon forms in topsoils and plant silicon uptake

Silicon (Si) enhances the stress resistance of rice plants. Silicon cycling in paddy fields is, however, still poorly studied. We examined relationships between Si forms in topsoil and plant Si uptake for four Vietnamese and three Philippine regions (ten fields per region). Mean rice straw Si concentrations within regions ranged from 3.0 to 8.4 %. For most of the Vietnamese fields they were lower than the critical value of 5.0 %, suggesting Si limitation of plant growth. For fields with low Si availability, straw Si concentrations were positively related to acetate-extractable Si in topsoil (i.e., dissolved and adsorbed Si). Such a relationship was not found for fields with high Si availability, presumably due to a maximum Si uptake capacity of rice plants. Mean annual Si uptake by rice within regions ranged from 0.31 to 1.40 Mg Si ha^?1 year^?1. They are determined by the continuous supply of plant-available Si during the cropping season and by aboveground biomass production. Weatherable silicate minerals mainly determine spatial differences in supply of plant-available Si. Concentrations of alkaline carbonate-extractable Si in topsoil (an estimate of amorphous Si) largely differed between regions; (regional means of 2.2–16.7 g Si kg^?1). The differences in concentrations and amounts in topsoil are not related to phytolith (i.e., amorphous Si in straw) input, presumably due to other yet uncertain factors on carbonate-extractable Si in soil (e.g., differences in phytolith solubility or contribution of non-phytolith sources to Si in the extracts).