Relationship between soil δ¹⁵N, C/N and N losses across land uses in New Zealand

Abstract

Several of the major processes that result in N loss from soil (nitrification, ammonia volatilization, and denitrification) discriminate against ¹⁵N and fractionate the stable N isotopes, thus δ¹⁵N of ecosystem components has been suggested as an indicator of ecosystem N leakiness. This concept has been applied more successfully to native systems (primarily forest) than to managed systems where N inputs are greater and N cycling processes have potentially been modified. We analysed 210 New Zealand soils (0–100 mm depth) from different land-use systems (increasing in intensity of land use management from indigenous, to plantation forestry, pasture under drystock, pasture under dairy, and cropping) for δ¹⁵N and measures of N availability (total N, C/N, and N mineralization) to determine whether increasing intensity of land use management would lead to increased soil δ¹⁵N values. Mean soil δ¹⁵N differed between land uses with intensively managed cropping having the highest mean soil δ¹⁵N (6.2‰) followed by dairy (5.4‰), drystock (3.8‰), forestry (2.8‰) and indigenous forests (2.1‰). Over all land uses there was a negative correlation between δ¹⁵N and the soil C/N ratio (ρ = −0.73) and regression analysis indicated a relatively strong linear relationship between δ¹⁵N and C/N (r² = 0.56, P < 0.001) when cropping sites (where significant loss of soil C had occurred), and sites with C/N ratio >18 were excluded. Typical N balances for each land use showed that total N loss (and in particular fractionating N losses from ammonia volatilization and nitrate leaching) also increased with increasing land-use intensity. Our results indicate that soil δ¹⁵N may be a useful tool in assessing potential N losses in different soils.