In situ core methods for estimating soil mineral-N fluxes: Re-evaluation based on 25 years of application and experience

A paper published in this journal 25 years ago described a new method for estimating the dynamics of soil mineral-N in situ. The method has been widely used, modified, evaluated and sometimes criticised, and highly cited. In this review we critically evaluate the utility of the method and the proposed modifications and summarize new insights gained over the past quarter of a century. There is no single recipe for the application of in situ methods to the study of soil mineral-N dynamics. Site specific factors need to be taken into account and some field testing is essential to fine-tune the method and to improve the reliability of estimates. Special care is required in soils that are rapidly nitrifying and those with poor drainage or where soil moisture levels can change rapidly. The use of appropriate and flexible periods of field incubation is essential. In soils with very low rates of net N mineralization (<10–15 kg N/ha), there is potential for a significant fraction of the total plant uptake of mineral-N to occur via mycorrhizal fungi during N turnover (i.e. the simultaneous processes of mineralization/immobilization). As a consequence, plants may acquire more mineral-N than the plant N uptake estimated from in situ soil measurements.There is considerable information available that can be used to evaluate the utility of in situ methods. Direct evidence includes: comparisons of estimated with measured uptake of N by vegetation; comparisons of estimated with measured leaching of N; and the budgeting of N added in fertilizer. Indirect evidence includes: quantitative linkages in forests of soil mineral-N dynamics to physiological processes such as the re-translocation of N in plant tissues, N cycling and forest productivity; and comparisons of modelled soil mineral-N dynamics with estimates obtained from in situ methods. We conclude that when the method is applied carefully and based on insights obtained from prior field testing, it produces reliable quantitative estimates (as opposed to merely being an index) of the soil mineral-N flux attributable to net N mineralization, uptake of N by vegetation and leaching of N in agricultural, managed forest and natural ecosystems. Soil mineral-N fluxes ranging from <10 to >150 kg N/ha/yr have been quantified using the method. The method is sufficiently sensitive to detect small short-term (monthly to seasonal) change as well as the long-term (annual or longer time scale) cumulative effects of management on soil mineral-N dynamics.Given the resource-demanding nature of their application, in situ methods are a research tool rather than methods that can be used routinely to better guide the management of N cycles. The method is valuable for: building an understanding of N cycling and how this relates to the functioning of natural and managed ecosystems; calibrating and testing simple indices of rates of N cycling; and for developing models that estimate soil N dynamics. In the future, more attention should be given to using in situ estimates of soil mineral-N dynamics to calibrate and test predictive models. Such models are vital to guiding improved management of N cycles, so as to achieve sustained efficient production whilst reducing adverse environmental impacts.