Interpretation of soil mineral nitrogen by scoring organic matter and nitrogen management as an “N-score” in the fields of Hokkaido before sugar beet planting

The objective of the present study was to investigate soil mineral nitrogen (N) in agricultural fields and to evaluate the factors that affect it. The amount of soil mineral N (0–100 cm depth) in 33 farming fields in the Hokkaido area, Japan, was investigated in April 2003, 2004 and 2005 (i.e. before sugar beet planting). The mean value and range of soil mineral N were 147 kg ha−1 and 44–750 kg ha−1, respectively. By analyzing six typical fields in detail (soil mineral N: 44–50 kg ha−1), we found that the history of organic matter application and other field management practices drastically affected the amount of soil mineral N. We referred to these histories and practices as “N-scores”. These scores take into account organic matter application and other field management practices by scoring N input using the Hokkaido Fertilizer Recommendations 2002. Soil mineral N was more closely related to the N-score (r = 0.839**, P < 0.01) than to other soil factors (e.g. pH, total C, total N, cation exchange capacity and autoclaved N). These results suggest that valuating organic matter application and other field management practices by scoring N input (N-score) is useful not only for evaluating soil N fertility, which is expressed by soil mineral N, but also for estimating the risk of N leaching.