Impact of harvest residues on soil mineral nitrogen dynamics following clearfall harvesting of a hoop pine plantation in subtropical Australia

Abstract

An alternative management strategy allowing post-harvest residues to remain as a blanket cover instead of being incorporated into windrows may prevent problems associated with lack of soil fertility in the inter-windrow space. Parallel, 2-year, in situ nitrogen (N) mineralisation studies were undertaken during the inter-rotation period following clearfall harvesting of a first-rotation hoop pine (Araucaria cunninghamii Aiton ex D. Don) plantation in subtropical Australia. We investigated the dynamics of ammonium N, nitrate N and nitrite N in the top 20 cm soil under a residue retention situation and under normal, operational conditions. Initially, ammonium N was the dominant form of soil mineral N but declined to <10 kg N ha−1 at both sites after 12 sampling cycles of 28-day duration. Nitrate N levels remained at approximately 30 kg N ha−1 despite seasonal fluctuations, throughout the 2-year sampling period. At the residue site there was no net N mineralisation in the soil until the 20th sampling cycle; approximately 110 kg N ha−1 was mineralised at the end of the sampling period. This compared with approximately 300 kg N ha−1 mineralised at the operational site for the same period. Approximately 100 and 220 kg N ha−1 were lost through leaching at the residue and operational sites, respectively. Residue retention promoted N immobilisation, reducing the potential for N losses through leaching and denitrification in the critical inter-rotation and early establishment period.