Evaluation of Soil Tests for Predicting Nitrogen Mineralization in Temperate Grassland Soils

Abstract

Improving nitrogen (N) fertilizer-use efficiency (NfUE) in temperate grassland soils is important for agronomic and environmental sustainability. Nitrogen supplied from soil N reserves through N mineralization processes need to be accounted for when prescribing additional N fertilizer applications. Therefore a rapid and reliable soil N testing procedure for predicting mineralizable N (MN) needs to be identified for temperate grassland soils. In this study, the MN across 35 mineral grassland soils (depth 0–10 cm) from Ireland, with varying physiochemical properties, was investigated using a 7-d anaerobic incubation (AI-7) as a reference method. The soil N pools measured using seven relatively rapid chemical N indices were also examined and compared with the AI-7 values. The Illinois Soil N Test (ISNT), total N (TN), and total C (TC) correlated strongly with MN, (r > 0.81) across these 35 soils. There was no distinct improvement when soils were grouped according to soil textural class. Although TN and TC displayed strong relationships with MN, these N indices may be less sensitive to short-term changes due to soil management practices and climates and consequently may be less suitable as an indicator. The ISNT was the best predicator of MN, explaining 69% of the variability in MN, suggesting that it was measuring a similar labile soil organic N pool to that released by AI-7. The ISNT shows potential to replace AI-7 as a more rapid and reliable soil N test for routine analysis across temperate soil types, however, further field based studies are required.