Nitrogen dynamics, apparent mineralization and balance calculations in a maize – wheat double cropping system of the North China Plain

• Adjusting fertilization to the crop requirements reduces the N balance in a farmer's field.
• N mineralized in spring is only utilized by the following summer-crop.
• N application above crop requirements increases the amount of cycling N and apparent N loss.

The aim of this study is to observe the N balance of a summer-maize/winter-wheat double cropping system of the North China Plain (NCP), using data from a static field experiment that was conducted in a previously over-fertilized farmer's field. Two reduced N treatments that were fertilized by adjusting N supply to crop N demand (UREA: urea; ASNDMPP: ammoniumsulphate nitrate + 3,4,-dimethylpyrazolephoshate) are compared to common farmers’ practice (FP: urea, 550 kg N ha−1 a−1) and to a control treatment (CK). Further, this research aims to estimate the importance of N mineralization for N supply in an intensive maize/wheat cropping system in order to better understand seasonal N dynamics in an over-fertilized system.The results of the experiment show that the N surplus (fertilized N − grain N) as well as the N balance (N Input − N Output) after harvest are significantly lower for the optimized treatments (Surplus: −25 kg to 98 kg N ha−1; Balance: −36 to 102 kg N ha−1) compared to FP (Surplus: 156 kg to 187 kg N ha−1; Balance: 56–262 kg N ha−1). This leads to lower residual N in the soil horizon from 0 to 90 cm in the reduced treatments (113 kg N ha−1 at end of experiment) compared to FP (293 kg N ha−1).Mineralization of N, which occurs mainly in the spring vegetation period of wheat and during the summer vegetation period of maize, plays a substantial role in N supply to the summer-crop. Apparent net N mineralization of the CK treatment was 161 kg N ha−1 for the first vegetation period of maize in 2009, decreasing to 27 kg N ha−1 over the following two vegetation periods. The two intermittent periods of winter-wheat showed an apparent net mineralization of 64 and 84 kg N ha−1, that did not influence yield formation. It is therefore likely that N mineralized during the spring vegetation period is carried over to the following vegetation period of summer-maize. In contrast to the CK treatment, an apparent net N loss was determined for all vegetation periods of summer maize in the FP treatment and for the first two vegetation periods of maize in the reduced treatments, even though mineralization in excess of the CK treatment was observed in an in situ mineralization experiment.The results show that the N balance in previously over-fertilized farmers’ fields of the NCP can be reduced by estimating crop N demand. Further, mineralization of N is an important factor both for N supply of crops, as well as for the loss of N during the summer vegetation periods and must be taken into consideration if N application rates in the NCP should be further reduced.