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

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 δ 15N may be a useful tool in assessing potential N losses in different soils.