Effects of straw returning levels on carbon footprint and net ecosystem economic benefits from rice-wheat rotation in central China.

Straw returning usually gives rise to greenhouse gas (GHG) emissions from the soil, and thus negatively affects carbon footprint (CF) of crop production. Numerous studies reported the effects of straw returning on the CF from single crop production. However, little is known about the integrated effects of different levels of straw returning on the CF and net ecosystem economic benefits (NEEB) from rice-wheat rotation. Here, we investigated the effects of different amounts of straw returning on soil CH4 and N2O emissions, GHG emissions from agricultural inputs (AIGHG), CF, and NEEB from a 2-year cycle of rice-wheat rotation. The CF was determined based on the total GHG emissions associated with crop production inputs and services. Overall, straw returning significantly increased annual CH4 emissions by 5.4-72.2% and reduced annual N2O emissions by 3.3-31.4% compared with straw removal. Straw returning remarkably increased rice grain yields by 8.1-9.9% and wheat grain yields by 10.2-21.1% compared with straw removal. The average annual AIGHG from rice-wheat rotation ranged from 3579 to 4987 kg CO2-eq ha-1. Diesel consumption played a dominant role in the AIGHG. The annual CF ranged from 0.96 to 1.31 kg CO2-eq kg-1 and increased with increasing straw returning amounts. The NEEB, which ranged from 14161 to 17413 CNY ha-1, was significantly affected by the levels of straw returning. The treatment with returning of 1/3 of preceding crop straw to the field (2.19-2.47 kg ha-1 year-1 of rice straw in the wheat season and 1.38-1.68 kg ha-1 year-1 of wheat straw in the rice season) resulted in relatively higher grain yield, the lowest CF, and the highest NEEB among all treatments, and thus can reduce CF, and increase grain yields and NEEB, and thus can be recommended as a sustainable approach to mitigate GHG emissions and increase economic benefits from rice-wheat rotation.

Integrated rice-duck farming decreases global warming potential and increases net ecosystem economic budget in central China.

Over the past decades, many attempts have been made to assess the effects of integrated rice-duck farming on greenhouse gas emissions, use efficient of energy, soil fertility, and economic significance. However, very few studies have been focused on the effects of the farming on net ecosystem economic budget (NEEB). Here, a 2-year field experiment was conducted to comprehensively investigate the effects of ducks raised in paddy fields on CH4 and N2O emissions, global warming potential (GWP), rice grain yield, and NEEB in central China. The experiment included two treatments: integrated rice-duck farming (RD) and conventional rice farming (R). The introduction of ducks into the paddy fields markedly increased the rice grain yield due to enhanced tiller number and root bleeding rate. RD treatment significantly elevated the N2O emissions (p < 0.05) but decreased CH4 emissions (p < 0.05) during rice growing seasons compared with R treatment. Analysis of GWP based on CH4 and N2O emissions showed that compared with R treatment, RD treatment significantly decreased the GWP by 28.1 and 28.0% and reduced the greenhouse gas intensity by 30.6 and 29.8% in 2009 and 2010, respectively. In addition, RD treatment increased NEEB by 40.8 and 39.7% respectively in 2009 and 2010 relative to R treatment. Taken together, our results suggest that the integrated rice-duck farming system is an effective strategy to optimize the economic and environmental benefits of paddy fields in central China.

Effects of Conservation Tillage on Topsoil Microbial Metabolic Characteristics and Organic Carbon within Aggregates under a Rice (Oryza sativa L.)-Wheat (Triticum aestivum L.) Cropping System in Central China.

Investigating microbial metabolic characteristics and soil organic carbon (SOC) within aggregates and their relationships under conservation tillage may be useful in revealing the mechanism of SOC sequestration in conservation tillage systems. However, limited studies have been conducted to investigate the relationship between SOC and microbial metabolic characteristics within aggregate fractions under conservation tillage. We hypothesized that close relationships can exist between SOC and microbial metabolic characteristics within aggregates under conservation tillage. In this study, a field experiment was conducted from June 2011 to June 2013 following a split-plot design of a randomized complete block with tillage practices [conventional intensive tillage (CT) and no tillage (NT)] as main plots and straw returning methods [preceding crop residue returning (S, 2100-2500 kg C ha-1) and removal (NS, 0 kg C ha(-1))] as subplots with three replications. The objective of this study was to reveal the effects of tillage practices and residue-returning methods on topsoil microbial metabolic characteristics and organic carbon (SOC) fractions within aggregates and their relationships under a rice-wheat cropping system in central China. Microbial metabolic characteristics investigated using the Biolog system was examined within two aggregate fractions (>0.25 and <0.25 mm). NT treatments significantly increased SOC concentration of bulk soil, >0.25 aggregate, and <0.25 mm aggregate in the 0-5 cm soil layer by 5.8%, 6.8% and 7.9% relative to CT treatments, respectively. S treatments had higher SOC concentration of bulk soil (12.9%), >0.25 mm aggregate (11.3%), and <0.25 mm aggregate (14.1%) than NS treatments. Compared with CT treatments, NT treatments increased MBC by 11.2%, 11.5%, and 20%, and dissolved organic carbon (DOC) concentration by 15.5%, 29.5%, and 14.1% of bulk soil, >0.25 mm aggregate, and <0.25 mm aggregate in the 0-5 cm soil layer, respectively. Compared with NS treatments, S treatments significantly increased MBC by 29.8%, 30.2%, and 24.1%, and DOC concentration by 23.2%, 25.0%, and 37.5% of bulk soil, >0.25 mm aggregate, and <0.25 mm aggregate in the 0-5 cm soil layer, respectively. Conservation tillage (NT and S) increased microbial metabolic activities and Shannon index in >0.25 and <0.25 mm aggregates in the 0-5 cm soil layer. Redundancy analysis showed that the SOC and its fractions (DOC and MBC) were closely correlated with microbial metabolic activities. Structural equation modelling showed that the increase in microbial metabolic activities directly improved SOC by promoting DOC in >0.25 mm aggregate in the upper (0-5 cm) soil layer under conservation tillage systems, as well as directly and indirectly by promoting DOC and MBC in <0.25 mm aggregate. Our results suggested that conservation tillage increased SOC in aggregates in the topsoil by improving microbial metabolic activities.

The effects of rape residue mulching on net global warming potential and greenhouse gas intensity from no-tillage paddy fields.

A field experiment was conducted to provide a complete greenhouse gas (GHG) accounting for global warming potential (GWP), net GWP, and greenhouse gas intensity (GHGI) from no-tillage (NT) paddy fields with different amounts of oilseed rape residue mulch (0, 3000, 4000, and 6000 kg dry matter (DM) ha(-1)) during a rice-growing season after 3 years of oilseed rape-rice cultivation. Residue mulching treatments showed significantly more organic carbon (C) density for the 0-20 cm soil layer at harvesting than no residue treatment. During a rice-growing season, residue mulching treatments sequestered significantly more organic C from 687 kg C ha(-1) season(-1) to 1654 kg C ha(-1) season(-1) than no residue treatment. Residue mulching significantly increased emissions of CO2 and N2O but decreased CH4 emissions. Residue mulching treatments significantly increased GWP by 9-30% but significantly decreased net GWP by 33-71% and GHGI by 35-72% relative to no residue treatment. These results suggest that agricultural economic viability and GHG mitigation can be achieved simultaneously by residue mulching on NT paddy fields in central China.

[Dynamics of active organic carbon in a paddy soil under different rice farming modes].

A field experiment was conducted to study the dynamics of dissolved organic carbon (DOC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC) in a paddy soil under integrated rice-duck farming (RD), intermittent irrigation (RW), and conventional flooded irrigation (CK), the three rice farming modes typical in southern China. Under these three farming modes, the soil DOC and MBC contents reached the highest during the period from rice booting to heading, while the soil ROC content had less change during the whole rice growth period. Two-factor variance analysis showed that soil MBC was greatly affected by rice growth stage, soil DOC was greatly affected by rice growth stage and farming mode, and soil ROC was mainly affected by farming mode. Comparing with CK, RD significantly increased the soil DOC and ROC contents and their availability, while RW significantly decreased the soil DOC content and its availability but increased the soil ROC content and its availability. No significant differences were observed in the soil MBC and microbial quotient among RD, RW, and CK.

[Effects of no-tillage and fertilization on paddy soil CH4 and N2O emissions and their greenhouse effect in central China].

By using static chamber-gas chromatographic techniques, the CH4 and N2O emissions from the paddy soil in southeast Hubei were measured. Four treatments were installed, i.e., no-tillage plus no-fertilization (NT0), conventional tillage plus no-fertilization (CT0), no-tillage plus fertilization (NTC), and conventional tillage plus fertilization (CTC). In all treatments, the CH4 emission had a seasonal variation of increasing first and decreasing then, while the N2O emission had no significant seasonal variation. Fertilization increased the CH4 and N2O emissions significantly. NT0 increased the CH4 emission and decreased the N2O emission significantly, compared with CT0; NTC only decreased the CH4 emission and increased the N2O emission slightly, compared with CTC. The analysis on the integrated greenhouse effect of CH4 and N2O showed that NT0 increased the effect by 25.9%, compared with CT0, while NTC decreased the effect by 10.1%, compared with CTC. Therefore, a reasonable arrangement of fertilization and no-tillage could reduce the integrated greenhouse effect of CH4 and N2O from paddy field.

[Effects of rice-duck farming on paddy field's methane emission].

By using closed-chamber technique, the characteristics of CH4 emission from an integrated rice-duck ecosystem were studied in 2006 and 2007. The results showed that comparing with conventional flooded rice ecosystem (CK), integrated rice-duck system (RD) was distinguished in its high dissolved oxygen (DO) in field surface water and significantly lower CH4 emission, due to the activities of duck. In 2006, the average CH4 emission flux and the total CH4 emission during the whole rice growth period of RD were 6.84 +/- 1.49 mg +/- m(-2) x h(-1) and 19.34 +/- 1.15 g x m(-2), being 32.7% and 26.3% lower than those of CK (10.17 +/- 1.25 mg x m(-2) x h(-1) and 26.25 +/- 2.17 g respectively; while in 2007, the values of the two test items of RD were 7.68 +/- 0.74 mg x m(-2) x h(-1) and 18.41 +/- 1.05 g x m(-2), being 19.0% and 19.3% lower than those of CK (9.53 +/- 0.40 mg x m(-2) x h(-1) and 22.81 +/- 0.75 g +/- m(-2)), respectively. The emission flux of CH4 had two peaks, being appeared at tillering and heading stages, respectively. The seasonal fluctuations of CH4 emission flux had significant positive correlations with soil temperature and soil dissolved organic carbon content (DOC), but no obvious correlation with soil total organic carbon.