The role of 15N in tracing N dynamics in agro-ecosystems under alternative systems of tillage management: A review

Cultivation has led to a substantial loss of soil organic matter in the surface soil layers. The use of no-till (or zero-till) has been promoted as a practice that maximises the retention on organic residues to reverse or slow the loss of soil organic matter. However, the terms are misnomers because sowing the crop disturbs the soil. The definitions of conservation tillage systems should be used, including quantifying the mass of residue return to the system. Over 116 research papers that used N-15 natural abundance, N-15 depleted or N-15 enriched isotopic techniques were reviewed to quantify the impact of tillage, especially no-till, on the N dynamics of cropping systems. The negative effects of no-till on crop yields at low fertiliser N applications were reported in numerous studies that had operated for short periods, with a few conducted on a long-term basis (> 10-15 years). Based on the cumulative research, we concluded that no-till had little impact on N-15 fertiliser recovery, or the accumulation of N released from the mineralisation of "native" soil organic matter. In most studies, the impact of no-till on N dynamics is confounded by soil water and soil temperature differences. Often the positive response to no-till was attributed to greater plant available water during the growing season. Soil moisture was the main determinant of the gross N transformation rate and the potential for N mineralisation was not different between conventional and no-till systems. The effect of no-till is restricted to the surface soil. Although differences exist in the surface soil, we were unable to confirm from the N-15 studies that no-till increased organic N sequestration. Net N mineralisation of the large unlabelled soil organic N pool in the root zone dilutes the effect of increased organic N accumulation and N mineralised in the surface layers of no-till systems. Tillage systems did affect the percentage of legume N derived from the atmosphere. Nitrogen fixation was greater under no-till compared to conventional cultivation when (i) mineral N was lower, and (ii) plant available water status was better due to suppression of weeds.