Changes in soil carbon stock under the wheat-based cropping systems at Vojvodina province of Serbia

The aim of this study was to assess the changes in soil organic carbon (SOC) stock in relation to the carbon (C) input from nine wheat-based cropping systems and untilled grass. The SOC pool ranged from 32.1 to 49.4 Mg ha^?1 at 0–20 cm and from 94 to 171 Mg ha^?1 at 0–100 cm for the arable soil, while in untilled grassland, it was higher (54 and 185 Mg C ha^?1, respectively). SOC stock was observed to be lower at the unfertilized 2-year rotation and higher at the 4-year rotation with manure and mineral fertilization. The study showed a winter wheat yield decrease of 176.8 kg ha^?1 for a 1- Mg ha^?1 SOC stock change in the 0–20-cm soil depth. The estimated C input for SOC stock maintenance was from 266 to 340 g C m^?2 year^?1 for winter wheat and rotations, respectively. Additional C input did not increase the SOC pool, suggesting that arable plots had a limited ability to increase SOC. These results provide guidance for the selection of management practices to improve C sequestration.     

Manure application has an effect on the carbon budget of a managed grassland in southern Hokkaido,Japan

Abstract

The carbon (C) budget of managed grassland in a cool-temperate region of Japan was estimated using a combination of eddy covariance and the biometric method for five years, to evaluate the effect of manure application. Chemical fertilizer was applied to the fertilizer (F) plot at a rate of 79 ± 20 kg N ha^?1 yr^?1. In the manure (M) plot, dairy cattle manure was applied at a rate of 10 Mg fresh matter ha^?1 yr^?1 (1923 ± 407 kg C ha^?1 yr^?1, 159 ± 68 kg N ha^?1 yr^?1). There was no significant difference in seasonal gross primary production (GPP) and harvest between the treatment plots, indicating that both fertilizer and manure can increase the biomass production. Annual net ecosystem production (NEP) and ecosystem respiration (RE) was significantly different between the treatment plots. The difference in RE, and between M and F plots approximates heterotrophic respiration of manure (RHm), which ranged from 0.9 to 1.3 Mg C ha^?1 yr^?1. Average annual RHm was 1.1 ± 0.4 Mg C ha^?1 yr^?1, and accounted for 56% of the total amount of applied manure C. The annual net biome production (NBP) in the M plot (from 0.0 to 1.5 Mg C ha^?1 yr^?1) was significantly higher than in the F plot (?1.4 to 0.5 Mg C ha^?1 yr^?1). The long-term effect of manure application combined with chemical fertilizer did not reduce grass production compared with chemical fertilizer only; however, manure application decreased the NEP throughout manure decomposition, and long-term manure application enhanced the NBP.     

Soil Air Permeability and Carbon Dioxide Flux Measurements from the Soil Surface

Abstract

Air permeability has been recognized as an index of soil structure and used in attempts to characterize soil pore geometry. The importance of increased carbon dioxide in soil to agriculture comes from the direct effects of carbon dioxide (CO₂) on root respiration of agricultural crops. In this study, the soil air permeability and CO₂ flux values were obtained using two different apparatuses built and designed to measure air permeability and CO₂ flux. Air permeability was obtained in clay soil using two different aggregate sizes. The average values obtained were 9.55×10^?8 and 1.78×10^?7 cm² for the <2-mm and 2- to 5-mm fractions, respectively. Carbon dioxide flux from the soil surface of no‐till and bare plots under winter conditions was measured using another apparatus. The average CO₂ flux for the no‐till plot was 2.88 g/m²‐day and for the bare plot was 1.31 g/m²‐day. These values were within the range of values obtained from other studies.     

Evaluation of the soil carbon budget under different upland cropping systems in central Hokkaido,Japan

Abstract

To evaluate the carbon budget in soils under different cropping systems, the carbon dioxide (CO₂) flux from soils was measured in a total of 11 upland crop fields within a small watershed in central Hokkaido over the no snow cover months for 3 years. The CO₂ flux was measured using a closed chamber method at bare plots established in each field to estimate soil organic matter decomposition. Temporal variation in instantaneous soil CO₂ fluxes within the sites was mainly controlled by soil temperature and moisture. Annual mean CO₂ fluxes and cumulative CO₂ emissions had no significant relationship with soil temperature and moisture (P > 0.2). However, there was a significant quadratic relationship between annual mean CO₂ flux or cumulative CO₂ emission and soil clay plus silt content (%) (R² = 0.72～0.74, P < 0.0003). According to this relationship, the optimum condition for soil CO₂ emission is at a clay plus silt content of 63%. The cumulative CO₂ emission during the no snow cover season within each year varied from 1,159 to 7,349 kg C ha^?1 at the different sites. The amount of crop residue carbon retained in the soils following a cropping season was not enough to offset the CO₂ emission from soil organic matter decomposition at all sites. As a consequence, the calculation of the soil carbon budget (i.e. the difference between the carbon added as crop residues and compost and the carbon lost as CO₂ from organic matter decomposition) ranged from –7,349 to –785 kg C ha^?1, except for a wheat site where a positive value of 4,901 kg C ha^?1 was observed because of a large input of organic carbon with compost. The negative values of the soil carbon budget indicate that these cropping systems were net sources of atmospheric CO₂.     

Grassland renovation increases N2O emission from a volcanic grassland soil in Nasu,Japan

Abstract

To investigate the effects of renovation (ploughing and resowing) on nitrous oxide (N₂O) emissions from grassland soil, we measured N₂O fluxes from renovated and unrenovated (control) grassland plots. On 22 August in both 2005 and 2006 we harvested the sward, ploughed the surface soil and then mixed roots and stubble into the surface soil with a rotovator. Next, we compacted the soil surface with a land roller, spread fertilizer at 40 kg N ha^?1 on the soil surface and sowed orchardgrass (Dactylis glomerata L., Natsumidori). In the control plot, we just harvested the sward and spread fertilizer. We determined N₂O fluxes for 2 months after the renovation using a vented closed chamber. During the first 2 weeks, the renovated plot produced much more N₂O than the control plot, suggesting that N was quickly mineralized from the incorporated roots and stubble. Even after 2 weeks, however, large N₂O emissions from the renovated plot were recorded after rainfall, when the soil surface was warmed by sunshine and the soil temperature rose 2.7–3.0°C more than that of the control plot. In 2005, during the 67-day period from 19 August to 26 October, the renovated and control plots emitted 5.3 ± 1.4 and 2.8 ± 0.7 kg N₂O-N ha^?1, with maximum fluxes of 3,659 and 1,322 µg N₂O-N m^?2 h^?1, respectively. In 2006, during the 65-day period from 21 August to 26 October, the renovated and control plots emitted 2.1 ± 0.6 and 0.96 ± 0.42 kg N₂O-N ha^?1, with maximum fluxes of 706 and 175 µg N₂O-N m^?2 h^?1, respectively. The cumulative N₂O emissions from plots in 2005 were greater than those in 2006, presumably because rainfall just after renovation was greater in 2005 than in 2006. These results suggest that incorporated roots and stubble may enlarge the anaerobic microsites in the soil in its decomposing process and increase the N₂O production derived from the residues and the fertilizer. In addition, rainfall and soil moisture and temperature conditions during and after renovation may control the cumulative N₂O emission.     

Impacts of nitrogen addition on the carbon balance in a temperate semiarid grassland ecosystem

Understanding carbon (C) cycling and sequestration in vegetation and soils, and their responses to nitrogen (N) deposition, is important for quantifying ecosystem responses to global climate change. Here, we describe a 2-year study of the C balance in a temperate grassland in northern China. We measured net ecosystem CO₂ exchange (NEE), net ecosystem production (NEP), and C sequestration rates in treatments with N addition ranging from 0 to 25 g N m^?2 year^?1. High N addition significantly increased ecosystem C sequestration, whose rates ranged from 122.06 g C m^?2 year^?1 (control) to 259.67 g C m^?2 year^?1 (25 g N). Cumulative NEE during the growing season decreased significantly at high and medium N addition, with values ranging from ?95.86 g C m^?2 (25 g N) to 0.15 g C m^?2 (5 g N). Only the highest N rate increased significantly cumulative soil microbial respiration compared with the control in the dry 2014 growing season. High N addition significantly increased net primary production (NPP) and NEP in both years, and NEP ranged from ?5.83 to 128.32 g C m^?2. The C input from litter decomposition was significant and must be quantified to accurately estimate NPP. Measuring C sequestration and NEP together may allow tracking of the effects of N addition on grassland C budgets. Overall, adding 25 or 10 g N m^?2 year^?1 improved the CO₂ sink of the grassland ecosystem, and increased grassland C sequestration.     

Effect of organic and inorganic sources of nutrients on soil microbial activity and soil organic carbon build-up under rice in west coast of India

The effect of medium-term (5 years) application of organic and inorganic sources of nutrients (as mineral or inorganic fertilizers) on soil organic carbon (SOC), SOC stock, carbon (C) build-up rate, microbial and enzyme activities in flooded rice soils was tested in west coast of India. Compared to the application of vermicompost, glyricidia (Glyricidia maculate) (fresh) and eupatorium (Chromolaena adenophorum) (fresh) and dhaincha (Sesbania rostrata) (fresh), the application of farmyard manure (FYM) and combined application of paddy straw (dry) and water hyacinth (PsWh) (fresh) improved the SOC content significantly (p < 0.05). The lowest (p < 0.05) SOC content (0.81%) was observed in untreated control. The highest (p < 0.05) SOC stock (23.7 Mg C ha^?1) was observed in FYM-treated plots followed by recommended dose of mineral fertilizer (RDF) (23.2 Mg C ha^?1) and it was lowest (16.5 Mg C ha^?1) in untreated control. Soil microbial biomass carbon (C_mb) (246 µg g^?1 soil) and C_mb/SOC (1.92%) were highest (p < 0.05) in FYM-treated plot. The highest (p < 0.05) value of metabolic quotient (qCO₂) was recorded under RDF (19.7 µg CO₂-C g^?1 C_mb h^?1) and untreated control (19.6 µg CO₂-C g^?1 C_mb h^?1). Application of organic and inorganic sources of nutrients impacted soil enzyme activities significantly (p < 0.05) with FYM causing highest dehydrogenase (20.5 µg TPF g^?1 day^?1), phosphatase (659 µg PNP g^?1 h^?1) and urease (0.29 µg urea g^?1 h^?1) activities. Application of organic source of nutrients especially FYM improved the microbial and enzyme activities in flooded and transplanted rice soils. Although the grain yield was higher with the application of RDF, but the use of FYM as an organic agricultural practice is more useful when efforts are intended to conserve more SOC and improved microbial activity.     

Effect of plant-mediated oxygen supply and drainage on greenhouse gas emission from a tropical peatland in Central Kalimantan,Indonesia

Abstract

To evaluate the hypothesis that plant-mediated oxygen supplies decrease methane (CH₄) production and total global warming potential (GWP) in a tropical peatland, the authors compared the fluxes and dissolved concentrations of greenhouse gases [GHGs; CH₄, carbon dioxide (CO₂) and nitrous oxide (N₂O)] and dissolved oxygen (DO) at multiple peatland ecosystems in Central Kalimantan, Indonesia. Study ecosystems included tropical peat swamp forest and degraded peatland areas that were burned and/or drained during the rainy season. CH₄ fluxes were significantly influenced by land use and drainage, which were highest in the flooded burnt sites (5.75 ± 6.66 mg C m^?2 h^?1) followed by the flooded forest sites (1.37 ± 2.03 mg C m^?2 h^?1), the drained burnt site (0.220 ± 0.143 mg C m^?2 h^?1), and the drained forest site (0.0084 ± 0.0321 mg C m^?2 h^?1). Dissolved CH₄ concentrations were also significantly affected by land use and drainage, which were highest in the flooded burnt sites (124 ± 84 μmol L^?1) followed by the drained burnt site (45.2 ± 29.8 μmol L^?1), the flooded forest sites (1.15 ± 1.38 μmol L^?1) and the drained forest site (0.860 ± 0.819 μmol L^?1). DO concentrations were influenced by land use only, which were significantly higher in the forest sites (6.9 ± 5.6 μmol L^?1) compared to the burnt sites (4.0 ± 2.9 μmol L^?1). These results suggest that CH₄ produced in the peat might be oxidized by plant-mediated oxygen supply in the forest sites. CO₂ fluxes were significantly higher in the drained forest site (340 ± 250 mg C m^?2 h^?1 with a water table level of ?20 to ?60 cm) than in the drained burnt site (108 ± 115 mg C m^?2 h^?1 with a water table level of ?15 to +10 cm). Dissolved CO₂ concentrations were 0.6–3.5 mmol L^?1, also highest in the drained forest site. These results suggested enhanced CO₂ emission by aerobic peat decomposition and plant respiration in the drained forest site. N₂O fluxes ranged from ?2.4 to ?8.7 μg N m^?2 h^?1 in the flooded sites and from 3.4 to 8.1 μg N m^?2 h^?1 in the drained sites. The negative N₂O fluxes might be caused by N₂O consumption by denitrification under flooded conditions. Dissolved N₂O concentrations were 0.005–0.22 μmol L^?1 but occurred at < 0.01 μmol L^?1 in most cases. GWP was mainly determined by CO₂ flux, with the highest levels in the drained forest site. Despite having almost the same CO₂ flux, GWP in the flooded burnt sites was 20% higher than that in the flooded forest sites due to the large CH₄ emission (not significant). N₂O fluxes made little contribution to GWP.     

Experimental Warming Effects on Soil Respiration,Nitrification, and Denitrification in a Winter Wheat-Soybean Rotation Cropland

A field experiment was conducted to examine responses of soil respiration, nitrification, and denitrification to warming in a winter wheat (Triticum aestivum L.)–soybean (Glycine max (L.) Merr) rotation cropland. The results showed that seasonal variations in soil respiration were positively related to seasonal fluctuations in soil temperature. Seasonal mean soil respiration rates for the experimental warming (EW) and control (CK) plots were 3.98 ± 0.43 and 2.54 ± 0.45 μmol m^?2 s^?1, respectively, in the winter wheat growing season, and they were 4.59 ± 0.16 and 4.36 ± 0.08 μmol m^?2 s^?1, respectively, in the soybean growing season. There was a marginally significant level (p = 0.097) for mean nitrification rates between EW and CK plots. Soil temperature and moisture accounted for 58.2% and 58.1% of the seasonal variations observed in the winter wheat and soybean plots, respectively.     

Soil Carbon and Nitrogen Fractions of a Grassland in Central Missouri,USA

Quantification of soil carbon (C) and nitrogen (N) fractions in grasslands is vital for estimating C sequestration and climate change studies. We quantified background soil total carbon (TOC) status, recalcitrant carbon (RC), acid hydrolysable labile carbon (AHC), hot- and cold-water extractable carbon (HWC and CWC, respectively) fractions in the grassland in this study. Soil C fractions were as follows: TOC (11,633 to 15,525 mg C kg^?1); RC (11,500 to 15,357 mg C kg^?1); AHC (132 to 168 mg C kg^?1); HWC (57 to 70 mg C kg^?1); and CWC (27 to 33 mg C kg^?1). Labile C fractions contributed at most 1.2% to total C. Concentrations of total N ranged from 1072 to 1230 g N kg^?1. Recalcitrant C contributed higher amounts (>90%) to total C, indicating the incorporation of C into the stable C fraction. Carbon dioxide (CO₂) and nitrous oxide (N₂O) fluxes were not significantly correlated with total C.     

Phosphorous and beneficial microorganism influence yield and yield components of wheat under full and limited irrigated conditions

Field experiment was conducted to investigate the impact of phosphorus (P) and beneficial microorganism (BM) on the yield and yield components wheat (Triticum aestivum L., cv. Siren-2010). The experiment was conducted under full (five irrigations) and limited (two) irrigation conditions at the Research Farm of The University of Agriculture Peshawar during winter 2012–13. The experiment under both full and limited irrigated conditions was laid out in randomized complete block design using three replications. The results showed that irrigated plots produced more spikes m^?2 (254), grains spike^?1 (55.5), heavier thousand grains weight (39.4 g), and higher grain yield (3612 kg ha^?1 than limited irrigated condition. Application of P at the highest rate (90 kg P ha^?1) produced more spikes m^?2 (260) and grains spike^?1 (52.4), and increased maximum thousand grain weight (39.1 g) and grain yield (3617 kg ha^?1). Application of BM at the highest rate (30 L ha^?1) resulted in maximum number of spikes m^?2 (257) and grains spike^?1(51.7), highest thousand grains weight (39.1 g) and grain yield (3765 kg ha^?1). The results confirmed that under full irrigated condition the increase in both P and BM levels (90 kg P ha^?1 and 30 L ha^?1, respectively) and under limited irrigated condition the intermediate levels of both P and BM (60 kg P ha^?1 and 20 L ha^?1, respectively) could increase wheat productivity under semi-arid conditions.     

Effects of grazing on CO2 balance in a semiarid steppe: field observations and modeling

Purpose

Carbon (C) dynamics in grassland ecosystem contributes to regional and global fluxes in carbon dioxide (CO₂) concentrations. Grazing is one of the main structuring factors in grassland, but the impact of grazing on the C budget is still under debate. In this study, in situ net ecosystem CO₂ exchange (NEE) observations by the eddy covariance technique were integrated with a modified process-oriented biogeochemistry model (denitrification–decomposition) to investigate the impacts of grazing on the long-term C budget of semiarid grasslands.

Materials and methods

NEE measurements were conducted in two adjacent grassland sites, non-grazing (NG) and moderate grazing (MG), during 2006–2007. We then used daily weather data for 1978–2007 in conjunction with soil properties and grazing scenarios as model inputs to simulate grassland productivity and C dynamics. The observed and simulated CO₂ fluxes under moderate grazing intensity were compared with those without grazing.

Results and discussion

NEE data from 2-year observations showed that moderate grazing significantly decreased grassland ecosystem CO₂ release and shifted the ecosystem from a negative CO₂ balance (releasing 34.00 g C?m^?2) at the NG site to a positive CO₂ balance (absorbing ?43.02 g C?m^?2) at the MG site. Supporting our experimental findings, the 30-year simulation also showed that moderate grazing significantly enhances the CO₂ uptake potential of the targeted grassland, shifting the ecosystem from a negative CO₂ balance (57.08?±?16.45 g C?m^?2?year^?1) without grazing to a positive CO₂ balance (?28.58?±?14.60 g C?m^?2?year^?1) under moderate grazing. The positive effects of grazing on CO₂ balance could primarily be attributed to an increase in productivity combined with a significant decrease of soil heterotrophic respiration and total ecosystem respiration.

Conclusions

We conclude that moderate grazing prevails over no-management practices in maintaining CO₂ balance in semiarid grasslands, moderating and mitigating the negative effects of global climate change on the CO₂ balance in grassland ecosystems.     

Effect of crop residue C:N ratio on N2O emissions from Gray Lowland soil in Mikasa,Hokkaido, Japan

Abstract

We studied the effect of crop residues with various C:N ratios on N₂O emissions from soil. We set up five experimental plots with four types of crop residues, onion leaf (OL), soybean stem and leaf (SSL), rice straw (RS) and wheat straw (WS), and no residue (NR) on Gray Lowland soil in Mikasa, Hokkaido, Japan. The C:N ratios of these crop residues were 11.6, 14.5, 62.3, and 110, respectively. Based on the results of a questionnaire survey of farmer practices, we determined appropriate application rates: 108, 168, 110, 141 and 0 g C m^?2 and 9.3, 11.6, 1.76, 1.28 and 0 g N m^?2, respectively. We measured N₂O, CO₂ and NO fluxes using a closed chamber method. At the same time, we measured soil temperature at a depth of 5 cm, water-filled pore space (WFPS), and the concentrations of soil NH⁺ ₄-N, NO^? ₃-N and water-soluble organic carbon (WSOC). Significant peaks of N₂O and CO₂ emissions came from OL and SSL just after application, but there were no emissions from RS, WS or NR. There was a significant relationship between N₂O and CO₂ emissions in each treatment except WS, and correlations between CO₂ flux and temperature in RS, soil NH⁺ ₄-N and N₂O flux in SSL and NR, soil NH⁺ ₄-N and CO₂ flux in SSL, and WSOC and CO₂ flux in WS. The ratio of N₂O-N/NO-N increased to approximately 100 in OL and SSL as N₂O emissions increased. Cumulative N₂O and CO₂ emissions increased as the C:N ratio decreased, but not significantly. The ratio of N₂O emission to applied N ranged from ?0.43% to 0.86%, and was significantly correlated with C:N ratio (y = ?0.59 ln [x] + 2.30, r ² = 0.99, P < 0.01). The ratio of CO₂ emissions to applied C ranged from ?5.8% to 45% and was also correlated with C:N ratio, but not significantly (r ² = 0.78, P = 0.11).     

Agronomic and Environmental Performance of Rapeseed Oilcake in the Lowland Rice Farming of Japan

The use of organic manures as an alternative to chemical fertilizers is an option for the development of sustainable farming systems in Japan. The present study examined the effect of rapeseed oilcake (RO) not only on yield and yield components of rice (Oryza sativa L.) but also on soil fertility parameters and surface water quality in a paddy field by comparing them with the effects of conventional inorganic fertilizer (IF) and a control (CR), that is, natural (unfertilized) system. The long-term field experiment from 1990 to 2006 demonstrated the indigenous sustainability of paddy systems, because husked grain yield and shoot biomass in the CR plot remained at moderate levels (ave. 323 ± 56 g m^?2 and 843 ± 166 g m^?2, respectively) over 17 successive years. Meanwhile, the application of RO comparatively improved grain yield (ave. 538 ± 83 g m^?2) and shoot biomass (ave. 1478 ± 187 g m^?2) to almost the same level as with IF (ave. 538 ± 68 g m^?2 and 1511 ± 173 g m^?2, respectively) during the study period. Soil fertility assessment in the last experimental year (2006) indicated that RO could improve the soil quality to the same extent as IF because major fertility parameters showed similar values between the RO and IF plots. On the other hand, the application of IF induced a substantial increase in estimated load outflows of nitrogen (N) and phosphorus (P) from the paddy surface water (2.46 g N m^?2; 0.38 g P m^?2) as compared to the CR plot (0.92 g N m^?2; 0.06 g P m^?2). Outflows from the RO plot showed relatively lesser effluent loads (2.00 g N m^?2; 0.14 g P m^?2) than the IF plot. From the findings of this study, we concluded that RO is agronomically similar to but an environmentally better resource material than conventional IF in the lowland rice farming system in Japan.     

Soil organic matter dynamics in lands continuously cultivated with maize and soybean in Heilongjiang Province,Northeast China: Estimation from natural 13C abundance

Abstract

Northeast China is the main production area of maize and soybean in China. In the present study, the rates of decomposition and replacement of soil organic carbon (SOC) were estimated using the soil inventory collected since 1991 from long-term maize and soybean cultivation plots in Heilongjiang Province, Northeast China, to evaluate the sustainability of the present cultivation system. The total carbon (C) content in soil was stable without any significant changes in the plots (approximately 28.5 g C kg^?1). The δ¹³C value of soil organic matter under continuous maize cultivation increased linearly with an annual increment of 0.07 from ?23.9 in 1991, which indicated that approximately 13% of the initial SOC was decomposed during the 13-year period of maize cultivation, with a half-life of 65 years. Slow decomposition of SOC was considered to result from the low annual mean temperature (1.5°C) and long freezing period (170–180 days year^?1) in the study area. In contrast, the amount of organic C derived from maize increased in the soil with a very slow annual increment of 0.17 g C kg^?1, probably because of the removal of all the plant residues from the plots. Based on the soil organic matter dynamics observed in the study plots, intentional recycling/maintenance of plant residues was proposed as a way of increasing soil fertility in maize or soybean cultivation.     

Regional evaluation of the spatio-temporal variation in soil organic carbon dynamics for rainfed cereal farming in northern Kazakhstan

Abstract

A regional evaluation of the soil organic carbon (SOC) dynamics for the chernozem zone in northern Kazakhstan is now vitally important for agricultural and environmental policy making. The objectives of the present study were: (1) to predict spatial and temporal variability in C input as crop residues using multi-temporal MODIS satellite images, (2) to clarify spatial and temporal variability in CO₂ emission as SOC output using geostatistics and model s, (3) to clarify spatial and temporal variability in the SOC budget using the results from (1) and (2). The mean growing-season C input as plant residues in cereal fields ranged from 0.9 to 1.4 Mg C ha^?1, with higher values in wet years. Carbon input as plant residues was higher in the northern part of the area than in the other parts. The average growing-season CO₂ emission ranged from 0.9 to 1.1 Mg C ha^?1, and was also higher in wet years than in dry years. In addition, more CO₂ was emitted in the northern part of this area. Accordingly the average growing-season C budget ranged from –0.2 to 0.3 Mg C ha^?1 and showed a negative correlation with air temperature during the crop-growing season. The 5-year C budget for different crop rotation systems ranged from –1.0 (3-year cropped cereal with 2-year bare fallow) to 0.4 (5-year continuous cereal cropped) Mg C ha^?1. These results indicate that fallow-based crop rotation systems are degradative with regard to the SOC budget in the studied area.     

Controlled irrigation and drainage of a rice paddy field reduced global warming potential of its gas emissions

The effect of controlled drainage on methane (CH₄) and nitrous oxide (N₂O) emissions from a paddy field under controlled irrigation (CI) was investigated by controlling the sub-surface drainage percolation rate with a lysimeter. CI technology is one of the major water-saving irrigation methods for rice growing in China. Water percolation rates were adjusted to three values (2, 5, and 8 mm d^?1) in the study. On the one hand, the CH₄ emission flux and total CH₄ emission from paddy fields under CI decreased with the increase of percolation rates. Total CH₄ emissions during the growth stage of rice were 1.83, 1.16, and 1.05 g m^?2 in the 2, 5, and 8 mm d^?1 plots, respectively. On the other hand, the N₂O emission flux and total N₂O emissions from paddy fields under CI increased with the increase of percolation rates. Total N₂O emissions during the growth stage of rice were 0.304, 0.367, and 0.480 g m^?2 in the 2, 5, and 8 mm d^?1 plots, respectively. The seasonal carbon dioxide (CO₂) equivalent of CH₄ and N₂O emissions from paddy fields under CI was lowest in the 2 mm d^?1 plot (1364 kg CO₂ ha^?1). This value was 1.4% and 19.4% lower compared with that in the 5 and 8 mm d^?1 plots, respectively. The joint application of CI and controlled drainage may be an effective mitigation strategy for reducing the carbon dioxide equivalents of CH₄ and N₂O emissions from paddy fields.     

Effect of Enhanced Nickel Levels on Wheat Plant Growth and Physiology under Salt Stress

A pot experiment was conducted to study the response of wheat to concurrent stresses of salinity and nickel (Ni)-toxicity. Soil was spiked with Ni (0, 20 and 40 mg kg^?1) combined with two levels of salinity (control and 10 dS m^?1) using Ni(ii) nitrate hexahydrate (Ni(NO₃)₂.6H₂O) and sodium chloride (NaCl), respectively, in a completely randomized design with four replicates. Plants were harvested at the tailoring stage and the results showed that wheat growth was positively affected by Ni at 20 mg kg^?1, and negatively at 40 mg kg^?1 concentration both in control and at 10 dS m^?1. Ni (20 mg kg^?1) had a nonsignificant positive effect on tissue potassium (K⁺) and a significant negative effect on Na⁺ concentration. Moreover, Ni translocation from root to shoot and accumulation decreased by increasing the levels of Ni in both control and salinity treatments. It can be concluded that Ni at 20 mg kg^?1 increased wheat growth by alleviating salinity stress; however, at 40 mg kg^?1 it aggravated the plant stress and decreased the plant growth.     

Root and Microbial Contributions to Soil Respiration during a Soybean Growing Season

Soil respiration is an important process for carbon geochemical cycling. Based on our five long‐term fertilizer experiments, soil respiration was measured using pot experiments with or without planting soybean. Soil respiration rates and soybean root biomass were determined at different observation times. Soil respiration rates due to soil microbial activity could be estimated by extrapolating a newly derived regressive equation at zero root biomass. Soil microbial respiration rates in the control were also observed directly, ranging from 16.0 to 42.7 mg carbon (C) m^?2 h^?1. Average soil microbial respiration rates from the regression analyses and direct observations were 32.9 and 27.8 mg C m^?2 h^?1, respectively. The average proportions of soil respiration rates due to the soybean growth were 63.0% using the regressive equation and 69.8% from direct observation. Therefore, the application of these two methods could provide new insight for separating plant root respiration from soil microbial respiration, which is important for estimating their individual contributions to atmospheric carbon dioxide.     

Spatial variation and interaction of runoff generation and erosion within a semi-arid, complex terrain catchment: a hierarchical approach

Purpose

Closed erosion plots have been used extensively to investigate soil loss and its spatial variation within a watershed. However, erosion rates measured on closed plots at various locations within a watershed may not reflect the “real world” conditions due to plot boundary problems. The purpose of this study was to identify runoff and sediment sources in a semi-arid, complex terrain catchment by using the data collected from open plots, nested catchments, and tunnel systems.

Materials and methods

The study catchment, in the Loess Plateau of China, was partitioned into various-level geomorphic units. Runoff and sediment discharges were measured from 55 storm events between 1963 and 1968 on open plots and nested catchments. Storm flows were also monitored in 14 rainfall events from the tunnel systems between 1989 and 1990. This study combined the data collected from the two periods to investigate runoff and sediment sources from the different geomorphic units of the catchment.

Results and discussion

On the four open plots (S1, S2, S3, and S4) of the hill slope, total runoff depths of 128.5 mm (S1), 84.3 mm (S2), 101.92 mm (S3), and 141.73 mm (S4) were recorded from all the events over the first period, which correspondingly produced total sediment yields of 3.056 kg m^?2 (S1), 9.058 kg m^?2 (S1), 42.848 kg m^?2 (S3), and 97.256 kg m^?2 (S4). The number of runoff events also varied due to a non-uniformity in runoff generation among the different geomorphic units of the catchment. Tunnel flows generally had higher mean sediment concentrations than catchment outflows. Three nested catchments located from the headwaters (C1) to the mouth of the catchment (C3) generated total runoff depths of 120.02 mm (C1), 143.92 mm (C2), and 149.43 mm (C3), and correspondingly produced sediments yields of 62.01 kg m^?2 (C1), 144.02 kg m^?2 (C2), and 123.92 kg m^?2 (C3) for the first period.

Conclusions

Significant variations in runoff and erosion existed within the catchment. The spatial variation of runoff generation on the hill slopes resulted from the variation of soil infiltration. Sediment produced from the lower hill slope zone was disproportionally higher than that from the upper hill slope zone. Nevertheless, a significant portion of the sediment eroded on the lower slope zone was caused by runoff generated from the upper slope zone. Tunnel erosion also played a significant role in sediment production.