Estimation of Annual Soil Nitrogen Mineralization Rates using an Organic‐Nitrogen Budget Approach

Abstract

Nitrogen (N) fertilizer use efficiency can be improved when nonfertilizer N inputs, including N mineralization from soil organic matter, can be quantified accurately. The objectives of this study were (i) to calculate annual N mineralization rates based on a budget for organic N inputs and outputs for soils under annual crops in California's Central Valley and (ii) to compare the results with temperature‐ and moisture‐adjusted N mineralization rates from a laboratory incubation. The organic‐N budget included organic N inputs from aboveground crop residues, roots, and root exudates as well as long‐term changes in soil organic N. A 10‐wk laboratory incubation was performed at 25°C and optimal moisture content with undisturbed soil cores collected pre‐plant in spring 2016 and 2017 from 30 field sites. The sites had no recent history of cover crops, manure, or compost applications. Based on the organic‐N budget, annual N mineralization rates in the top 30 cm of the profile ranged from 76 to 123 kg ha −1 yr −1 . Taking soil moisture and temperature in furrow‐irrigated fields into account, approximately half of this amount was mineralized during a cropping season from mid‐April to mid‐August. Even when corrected for daily temperature and moisture content, N mineralization rates in the incubation study were considerably higher than the rates estimated with the organic‐N budget. The organic‐N budget provides an estimate of the annual N mineralization rates in these soils. Adjustments for soil properties and crop management will likely improve estimates for individual sites. Core Ideas Annual N mineralization rates were calculated with an organic‐N budget. Calculated annual N mineralization rates ranged from 76 to 123 kg ha −1 . A laboratory incubation overestimated N mineralization.