Optimizing application of dairy effluent with synthetic N fertilizer reduced nitrogen leaching in clay loam soil

High application rates of dairy effluent and manure are often associated with nitrogen (N) leaching, which can affect groundwater quality. Here, we used a lysimeter to examine N leaching losses and biomass yield following application of dairy effluent and manure under wheat-maize cropping. The field experiment included seven treatments: no N fertilizer (Control); 200/300 kg N ha−1 synthetic N fertilizer only (wheat/maize) (CN); 100/150 kg N ha−1 synthetic N fertilizer plus 100/150 (DE1), 150/200 (DE2) and 250/350 (DE3) kg N ha−1 dairy effluent; 100/150 kg N ha−1 synthetic fertilizer plus 100/150 kg N ha−1 dairy manure (SM1); and 150/225 kg N ha−1 synthetic fertilizer plus 50/75 kg N ha−1 dairy manure (SM2). Compared with CN, DE1 treatment increased maize yield by 10.0 %, wheat N use efficiency (NUE) by 26.5 %, and wheat and maize N uptake by 7.7–16.3 %, while reduced N leaching by 22.4 % in wheat season and by 40.4 % in the maize season. In contrast, DE2 and DE3 treatment increased N leaching by 27.2–241 % and reduced NUE by 26.2–55.2 %. SM2 treatment increased yield and NUE by 8.8 % and 7.8 %, respectively, and reduced N leaching by 42.9 % during the wheat but not the maize season. Annual N leaching losses were 37.6 kg N ha−1 under CN treatment, but decreased to 27.4 kg N ha−1 under DE1. In contrast, N leaching increased to 52.8 and 84.1 kg N ha−1 under DE2 and DE3 treatment, respectively (P