Integrating beef cattle on cropland affects net global warming potential

Recent interest in integrated crop-livestock (ICL) systems has prompted numerous investigations to quantify ecosystem service tradeoffs associated with management. However, few investigations have quantified ICL management effects on net global warming potential (GWP), particularly in semiarid regions. Therefore, we determined net GWP for grazed and ungrazed cropland in a long-term ICL study near Mandan, ND USA. Factors evaluated for their contribution to net GWP included carbon dioxide (CO2) emissions associated with production inputs and field operations, methane (CH4) emissions from enteric fermentation by beef cattle, change in soil carbon stocks, and soil-atmosphere CH4 and nitrous oxide (N2O) fluxes. Net GWP was significantly greater for grazed cropland (946 kg CO2equiv. ha-1 yr-1) compared to ungrazed cropland (200 kg CO2equiv. ha-1 yr-1) (P=0.0331). The difference in net GWP between treatments was largely driven by emissions from enteric fermentation (602 kg CO2equiv. ha-1 yr-1). Among other contributing factors, CO2 emissions associated with seed production and field operations were lower under ungrazed cropland (P = 0.0015 and 0.0135, respectively), while soil CH4 uptake was greater under grazed cropland (P = 0.0102). Soil-atmosphere N2O flux from each system negated nearly all the CO2equiv. sink capacity accrued from soil carbon stock change. As both production systems resulted in net greenhouse gas (GHG) emissions to the atmosphere, novel practices that constrain GHG sources and boost GHG sinks under semiarid conditions are recommended.

Effect of long-term differentiated fertilisation regimes on greenhouse gas emissions from a subtropical rice-wheat cropping system

A field campaign was conducted using six treatments under the summer rice-winter wheat cultivation system to evaluate the response of soil greenhouse gas (GHG) emissions to long-term differentiated fertilisation regimes. The treatments included control, phosphorus plus potassium, nitrogen only, nitrogen plus phosphorus (NP), nitrogen plus potassium, and NP plus potassium (NPK). Compared to the control, mineral fertilisation increased CH₄ emissions during the rice season by 69% to 175%. Phosphorus amendment also enhanced seasonal CO₂ emissions by 21% to 34% when compared with the treatments without receiving P, while combined use of P and potassium suppressed seasonal N₂O emission to the same level of control. Net CO₂ and N₂O emissions from the dried fallow and wheat seasons and CH₄ emissions from the flooding rice season dominated annual budgets of individual GHGs. All of the soils under different treatments were net sources of global warming and the overall net global warming potential ranged from 9 799 to 14 178 kg CO₂ eq/ha/year with CO₂ emission contributing 52% to 76%, CH₄ contributing 20% to 40% and N₂O occupying the rest. The annual maximum grain yields and minimum GHG intensity was observed at the NPK treatment, suggesting it to be the environmental-friendly optimum fertilisation regime.

Strong mitigation of net global warming potential (GWP) via short-term aerobic pre-digestion of green manured soil in rice paddy

<p> Cover crop cultivation is strongly recommended during fallow season to increase soil organic carbon (SOC) stock. However, since its biomass recycling as green manure can dramatically increase greenhouse gas (GHG) emission, in particular, methane (CH<sub>4</sub>) during rice cropping season, smart cover crop management strategy should be developed. In our previous research, CH<sub>4</sub> emission during cropping season was dramatically reduced via short-term aerobic decomposition before irrigation (Lee et al.). However, due to a fast response rate of aerobic decomposition, the effect of mitigating CH<sub>4</sub> emission could be offset by SOC depletion which results in accelerating global warming. To evaluate the comprehensive impact of the short-term aerobic decomposition on global warming, net global warming potential (GWP), defined as the difference between GWP and SOC stock change was employed. SOC stock change was estimated using net ecosystem carbon budget (NECB), a balance between soil C input and output. The mixture of barley and hairy vetch cultivated during the dried fallow season, and then its whole biomass was incorporated 0-30 days before irrigation for rice transplanting. The aerobic decomposition of cover crop biomass significantly reduced CH<sub>4</sub> emission by 24-85% over control but negligibly influences N<sub>2</sub>O emission. Total C input and output were unaffected by the aerobic digestion. Although carbon emission before flooding dramatically increased after biomass application in aerobic decomposition treatments, the mineralized C losses exhibited no differences among treatments. Based on these results, NECB values were similar in all treatments. This implies the aerobic decomposition did not stimulate SOC depletion, compared to the control. Finally, the net GWP highly decreased by 30-86% by the aerobic digestion due to the significant reduction of CH<sub>4</sub> emission. In conclusion, earlier application of cover crops before irrigation is a smart strategy to decrease methane emission, maintaining soil carbon sequestration effect of cover crop biomasses application.</p>