SHORT-TERM NET N MINERALIZATION FROM PLANT RESIDUES AND GROSS AND NETN MINERALIZATION FROM SOIL ORGANIC-MATTER AFTER REWETTING OF A SEASONALLY DRY SOIL

The mineralization of N from shoot residues of two legume species and a common weed component of Western Australian pastures was measured af ter a simulated 'summer rainfall' in a laboratory experiment using und isturbed soil cores of a loamy sand. Water was added to the surface of the cores equivalent to a single rainfall event of 10 mm water (treat ment 1); or followed 48 h later by a further application of 10 mol wat er (treatment 2). Net mineralization and microbial biomass N were meas ured over 144 h following the initial rewetting of the soil cores. Thr ee types of N-15-labelled plant residue, namely (i) lupin leaf (Lupinu s angustifolium), (ii) clover (Trifolium subterraneum) shoot, burr and petiole, and (iii) capeweed (Arctotheca calendula) shoots and leaves were placed on the soil surface as large fragments and examined to ass ess their mineralization and incorporation into microbial biomass. The soils dried rapidly after rewetting and net mineralization of N was v ery low. The proportion of N as nitrate was increased in the rewetted soils with the major changes occurring in the top 5 cm of soil. Howeve r, there was very little mineralization of the surface-applied plant r esidues, with more than 90% of the mineral N being derived from the na tive organic matter. Microbial biomass showed fluctuations in both the rewetted treatments, but no consistent increase or decrease and no si gnificant immobilization of N-15. Gross N mineralization was measured using an isotopic dilution technique involving the injection of N-15-l abelled solutions into the soil. Estimates of gross N mineralization a fter a single rainfall event of 10 mm water were, on average, four tim es greater than measures of net N mineralization. Gross N mineralizati on rates declined as the soil dried, with three times more mineralizat ion occurring in the 0-5 cm depth (4.73-8.93 mu g g(-1) day(-1)), comp ared with the 5-10 cm depth (0.86-2.38 mu g g(-1) day(-1)). A major di sadvantage with the isotopic dilution method was that injection of the solutions into soil greatly increased the soil moisture content. Gros s mineralization in the injected cores is likely to have been overesti mated because of N movement below the sampling zone and increased micr obial activity relative to non-injected cores.