Effects of weed control and fertilization on soil carbon and nutrient pools in an exotic pine plantation of subtropical Australia

Purpose Soil carbon (C) and nutrient pools under different plantation weed control and fertilizer management treatments were assessed in a 7-year-old, F-1 hybrid (Pinus elliottii var. elliottii x Pinus caribaea var. hondurensis) plantation in southeast Queensland, Australia. This research aimed to investigate how early establishment silvicultural treatments would affect weed biomass, soil C, nitrogen (N) and other nutrient pools; and soil C (delta C-13) and N isotope composition (delta N-15) to help explain the key soil processes regulating the soil C and nutrient pools and dynamics. Materials and methods Soils were sampled in June 2006 in both the planting row and in the inter-planting row at three depths (0-5, 5-10, and 10-20 cm). Soil parameters including total and labile C and N pools; soil delta C-13 and delta N-15; total phosphorus (P); extractable potassium (K); moisture content and weed biomass were investigated. Results and discussion The luxury weed control treatments significantly reduced weed biomass and its organic residues returned to the soil in the first 7 years of plantation development. This resulted in significant variations at some depths and positions in soil delta C-13, delta N-15, extractable K, hot water extractable organic C (HWEOC), hot water extractable total N (HWETN), potentially mineralizable N (PMN), and soil moisture content (MC). Luxury weed control in the absence of luxury fertilization also significantly decreased extractable K. There was a significant interaction between soil depth and sampling position for soil total C, total N, HWEOC, and HWETN. Weed biomass correlated positively with soil total N, delta C-13, PMN, MC, HWEOC, and HWETN. Conclusions Luxury weed control treatments significantly reduced weed biomass leading to a reduction of soil organic matter. Soil delta C-13 and delta(15), together with the other soil labile C and N pools, were sensitive and useful indicators of soil C dynamics and N cycling processes in the exotic pine plantation of subtropical Australia.