Soil nitrogen mineralization and fate of (15NH4)2SO4 in field-incubated soil in a hardwood plantation of subtropical Australia: the effect of mulching

Background, aim, and scope Mulching is frequently used to overcome the drought problem in hardwood plantations that are increasingly being established in lower rainfall areas of Queensland, Australia because of increasing land values. In addition to soil water content, soil nitrogen (N) availability is another critical determinant of plantation productivity in these areas. The purpose of this study was to understand how soil mineral N dynamics, in situ N mineralization, and the fate of fertilized N would be affected by mulching during the early establishment of hardwood plantations. Materials and methods Two treatments (mulching, 1.57-kg m(-2) site preparation residues, and nonmulching) were applied to six 24-m-wide by 67.2-m-long plots at Pechey (27 degrees 18' S, 152 degrees 3' E) in southeast Queensland, Australia. At each sampling, two in situ N mineralization tubes were driven into the ground to a depth of 10 cm. One tube was immediately removed to provide baseline data for that sampling cycle. Another tube was left in situ for 1 month after which it was removed and a further two tubes were inserted to repeat the cycle. Sampling took place over 12 consecutive cycles, spanning one calendar year, commencing in January 2005. Between January and March 2005, another pair of soil tubes was driven into the ground to examine the fate of N-15-labeled fertilizer. Briefly, 20-mL N-15-labeled (NH4)(2)SO4 (10.3 at.% N-15) solution, containing 1.58 mg N, was uniformly applied on the soil surface of the tubes, giving 2.75 kg/ha N in each soil tube. Then, one tube was removed immediately and another left for 1 month as described above. Mineral N in all soil tubes was extracted with 2 M KCl. KCl extracts were analyzed colorimetrically for NH4+-N and NO3--N using a Lachat Autoanalyzer. The isotope ratio of mineral N and total N in N-15-labeled tubes was analyzed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer (Isoprime-EuroEA 3000). Results and discussion Mulching had no significant effect on net soil N mineralization during the 12-month period but had a significantly negative effect on net nitrification when net soil N mineralization was partitioned into net nitrification and net ammonification. Net nitrification constituted more than 85% of net N mineralization for both treatments. Nitrification was significantly correlated with air maximum temperature but poorly correlated with soil moisture. The results of unlabeled and N-15-labeled in situ N mineralization experiments showed that reduced nitrification and increased N immobilization resulting from mulching were the main reasons for the lower mineral N of mulched soil in the early establishment period of hardwood plantation in subtropical Australia. Conclusions, recommendations, and perspectives The field incubation results indicate that if soil is mulched with site preparation residues, net nitrification and hence mineral N are lower than nonmulched soil during the first 12-month establishment period of hardwood plantation in subtropical Australia. Increased soil nitrification in the nonmulching treatment means greater potential for N losses through leaching and denitrification. N immobilization as a result of mulching implied that more N was retained in the soil, which may benefit the trees in the longer term. However, long-term research may be necessary to substantiate the conclusion.