Agronomically significant N yield responses of cereals following grain legumes compared with cereal monoculture are frequently measured. The positive N response of the cereal has been attributed to the transfer of biologically fixed N, to N-sparing under the antecedent legume, and to less immobilisation of nitrate during the decomposition of legume residues. Methods for estimating the transfer of biologically fixed N in rotations, and for separating the N benefit into fixed N and non-fixed N components, are reviewed. Available data indicate that both sources of N contribute to the N benefit. The role of the grain legume in the gain or drain of soil N is evaluated by considering the balance between symbiotic dependence and N harvest index, as well as long-term changes in total soil N. Several 15N-based techniques for direct estimation of inputs of biologically fixed N to the soil N pool are reviewed. N balances in grain legume-cereal rotations may be positive or negative depending on the legume species, symbiotic performance, and agronomic factors.

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After decades of searching for a rapid method to estimate the N mineralization capacity of soil, there is still no consistent recommendation. It is legitimate to examine the causes for the often‐conflicting results in literature. The efficacy of various references that have been used as benchmarks for assessing chemical and physical indices in the literature is critically reviewed in this paper. Gross N mineralization and consumption during waterlogged and aerobic incubations were estimated in a wide range of soils. It was found that equivalent to 17 to 90 and 23 to 59% of the mineralized N was consumed during the waterlogged and aerobic incubations, respectively. As net N production rate represents the balance between N‐producing and N‐consuming processes, it appears difficult to find a simple method that could be used to predict the net effect of several concurrent processes. We used the gross N mineralization as a reference criterion for N mineralization ability. Total organic N, water‐soluble organic N, alkali‐hydrolyzable N, acid‐hydrolyzable N, hot salt‐hydrolyzable N and N in the light organic matter fraction were assessed against this reference criterion. All indices except light fraction N were significantly related to gross N mineralization. Water‐soluble organic N had the highest correlation of all the indices tested. None of the chemically hydrolyzed N fractions consistently showed closer relationships with N mineralization than total organic N, suggesting that these chemical methods are ineffective in extracting a biologically labile fraction of soil organic N.

Among the lighter elements having two or more stable isotopes (H, C, N, O, S), δ(15)N appears to be the most promising isotopic marker to differentiate plant products from conventional and organic farms. Organic plant products vary within a range of δ(15)N values of +0.3 to +14.6%, while conventional plant products range from negative to positive values, i.e. -4.0 to +8.7%. The main factors affecting δ(15)N signatures of plants are N fertilizers, biological N2 fixation, plant organs and plant age. Correlations between mode of production and δ(13)C (except greenhouse tomatoes warmed with natural gas) or δ(34)S signatures have not been established, and δ(2)H and δ(18)O are unsuitable markers due to the overriding effect of climate on the isotopic composition of plant-available water. Because there is potential overlap between the δ(15)N signatures of organic and conventionally produced plant products, δ(15)N has seldom been used successfully as the sole criterion for differentiation, but when combined with complementary analytical techniques and appropriate statistical tools, the probability of a correct identification increases. The use of organic fertilizers by conventional farmers or the marketing of organic produce as conventional due to market pressures are additional factors confounding correct identification. The robustness of using δ(15)N to differentiate mode of production will depend on the establishment of databases that have been verified for individual plant products.