Ryegrass early sowing into soybean to mitigate nitrous oxide emissions in a subtropical integrated crop-livestock system

Abstract

Ryegrass early sowing at the beginning of soybean grain maturation (stage R7) results in an earlier offering of forage for animals in integrated crop-livestock (ICL) systems. In addition, early sowing may decrease N2O emissions compared to ryegrass sowing after soybean harvest, although this possibility requires further study. We performed a two-year study to evaluate the effects of ryegrass early sowing in an ICL system under subtropical conditions. The following treatments were tested: (1) ryegrass early sowing into R7 soybean (ICL-I); (2) ryegrass sowing after soybean harvest (ICL-II); and (3) continuous cultivation for grain production with soybean/canola and soybean/wheat successions (CC). We measured the N2O fluxes, soil NH4+ and NO3− dynamics, grain yield and forage production. The soybean grain yield was not affected by the ICL and CC systems. Forage was offered earlier in the ICL-I system than in the ICL-II system over the two-year period. Ryegrass forage production was two times higher in the ICL-I system than in the ICL-II system during the first year (5.0 vs 2.3 Mg dry matter ha-1). Ryegrass early sowing efficiently decreased the soil NO3− content from soybean stage R7 until the end of the ryegrass cycle. Cumulative N2O emissions were positively correlated with nitrate intensity (R2 = 0.95), indicating that practices that decreased the soil NO3− accumulation, such as ryegrass early sowing, contributed to decreased N2O emissions. The mean cumulative N2O emissions for the two-year period was 18.5% lower for ICL-I than for ICL-II (821.3 vs 1007.2 g N-N2O ha-1). The cumulative N2O emissions were 24 and 21% lower for ICL than for CC for the first and second years, respectively. The present study shows that ryegrass early sowing is an alternative that can decrease annual N2O emissions and may contribute to improving the greenhouse gas balance in ICL systems compared to that in grain production systems.