Soil aggregation and greenhouse gas flux after 15 years of wheat straw and fertilizer management in a no-till system

Mulching effect on carbon (C) sequestration depends on soil properties, mulch material, and the rate and duration of application. Thus, rate of soil C sequestration was assessed on a 15 year field study involving three levels of wheat straw at 0 (M0), 8 (M8) and 16 (M16) Mg ha−1 yr−1, at two levels (244 kg N ha−1 yr−1, F1 and without, F0) of supplemental N. Soil C concentration was assessed in relation to aggregation and occlusion in aggregates of a silt loam Alfisol under a no-till (NT) and crop-free system in central Ohio. In comparison to control, soil organic carbon (SOC) concentration in the 0–10 cm depth of bulk soil increased by 32% and 90% with M8 and M16 treatments with a corresponding increase in the SOC stock by 21–25% and 50–60%, respectively. With increase in rate of residue mulch, proportion of water stable aggregates (small macroaggregates, >250 μm size) increased by 1.4–1.8 times and of microaggregates (53–250 μm) by 1.4 times. Fertilizer N significantly increased the SOC concentration of small macroaggregates under M16 treatments only. Ultra-sonication showed that 12–20% of SOC occluded in the inter-microaggregate space of small macroaggergates, was a function of both mulch and fertilizer rates. Significantly higher and positive correlation of greenhouse gases (GHGs), CO2, CH4 and N2O flux was observed with C and N concentrations of small macroaggregates and also of the occluded fraction of small macroaggregates. The higher correlation coefficient indicated the latter to be prone to microbial attack. On the contrary, non-significant relationship with C and N concentrations of microaggregates indicate a possible protection of microaggregate C. The diurnal fluxes of CO2, CH4 and N2O were the lowest under bare soil and the highest under high mulch rate with added N, with values ranging from 1.51 to 2.31 g m−2 d−1, −2.79 to 3.15 mg m−2 d−1 and 0.46 to 1.02 mg m−2 d−1, respectively. Mulch rate affected the GHGs flux more than did the fertilizer rates. The net global warming potential (GWP) was higher for high mulch (M16) than low mulch (M8) rates, with values ranging from 0.46 to 0.57 Mg CO2 equivalent – C ha−1 yr−1 (M8) and 1.98 to 3.05 Mg CO2 equivalent – C ha−1 yr−1 (M16). In general, mulch rate determined the effect of fertilizers. The study indicated that over long-term, a mulch rate between 8 and 16 Mg ha−1 yr−1 may be optimal for Alfisols in Central Ohio.