Contrasting impacts of long-term application of manure and crop straw on residual nitrate-N along the soil profile in the North China Plain

The effects of long-term animal manure application and crop straw incorporation on the migration of carbon (C) and nitrogen (N) deep into the soil profile and the associated N leaching risk in particular have not been thoroughly elucidated to date. Soil profile samples were collected from depths of up to 200 cm from the following four treatments in a 27-year field experiment on the North China Plain: N + phosphorus (P) + potassium (K) fertilizers (NPK), NPK + 22.5 t ha(-1) swine manure (NPKM), NPK + 33.7 t ha(-1) swine manure (NPKM+) and NPK + straw incorporation (NPKS). The results revealed that long-term manure application and straw incorporation significantly enhanced the soil organic C (SOC) and total N (TN) contents in the upper 20 cm and that this effect was weaker in the deeper soil layers (P < 0.05). Residual nitrate-N (NO3--N) contents at 0 to 40 cm and 120 to 200 cm in the NPKM and NPKM+ were 4-16 and 2-9 times higher than those in the NPK and NPKS, respectively. These results indicated a greater potential for N leaching from manure addition and a higher propensity for NO3--N leaching out of the 40-100 cm soil layer. Pearson relationship analysis demonstrated that NO3--N content was clearly affected by SOC and dissolved organic N (DON) contents along the soil profile (20-200 cm), implying that the higher residual NO3--N contents in the deeper soil from manure addition were partially attributable to the mineralization and nitrification of the downward SOC and DON. Interestingly, a low level of residual NO3--N combined with negative mineralization in the 120-200 cm soil layers of the NPKS treatment was observed, suggesting that straw incorporation promotes soil NO3--N retention. Thus, we concluded that long-term manure application is beneficial for soil NO3--N content retention, whereas long-term straw incorporation benefits NO3--N retention. (C) 2018 Elsevier B.V. All rights reserved.