The Effects of Different Harvest Intensities on the Distribution of Soil Phosphorus and Nutrient Stocks in a Brazilian Oxisol

Abstract

Forest soils form an important reservoir in ecosystem nutrient and carbon budgets, which are crucial for sustained productivity of forests. Nutrient fluxes and transformations in forest soils are a result of a complex interchange between the atmosphere, plants, and soil. Consequently, soils are a critical source of plant nutrition, and integral to the recovery of ecosystems following natural or human disturbances. As essential plant nutrients, the concentrations of plant available nitrogen (N), phosphorous (P), and sulfur (S) are crucial controls on the net primary productivity of most forest sites. As the plants uptake these nutrients, their concentrations within the soil are reduced until they are replaced either by nutrient recycling or fresh inputs. Highly weathered tropical soils are characteristically deficient in essential plant nutrients. This is especially true of the macronutrients essential for plant growth and development. Organic matter (OM) plays a fundamental role in soil fertility, contributing to cation exchange capacity (CEC) and as a source of nutrients, while also stabilizing soil aggregates and increasing water holding capacity. Soil management practices can sustain adequate levels of OM while providing sufficient quantities of plant available P. The need for balancing high timber productivity while maintaining soil nutrient concentrations has led to the development of sustainable silvicultural practices. Retaining forest harvest residues on the soil surface can help to return nutrients to the soil, as well as contributing substantial quantities of OM, thus aiding in a more sustainable production system. This research project identified the different forms of P in the soil (labile P, moderately labile P and non-labile P), as well as their concentrations at depth (to approximately two meters) in an Oxisol planted with Eucalyptus in Brazil under different timber harvest intensities. The harvest regimes considered were (i) conventional stem-only harvest (all forest residues were maintained on the soil); (ii) whole-tree harvest (only litter was maintained on the soil – all slash, stemwood and bark were removed) and (iii) whole-tree harvest + litter layer removal. In addition, this research seeks to establish the relationship between the different forms of P and carbon (C) and nitrogen (N) in the soil. A reduction in all nutrient stocks was observed in the 0-20 cm layer for all treatments. For N, this reduction was 20% higher when harvest residues were removed from the soil, and 40% higher when no N fertilizer was applied. The maintenance of harvest residues on the soil reduces the loss of N, P, and S due to harvest by 120%, 50%, and 40%, respectively. Our study has also shown that organic and inorganic P forms in the soil can act as source or sink to the soil solution, depending on the mineralogical composition of the soil, fertilization and harvest management applied. From 2004 to 2016, it was observed a reduction in the relative contribution of non-labile P in the treatments for which all harvest residues were maintained. However, this fraction of P still accounts for more than 60% of total P present in the soil, followed by labile and moderately labile forms of the nutrient. The highest stocks of total P were observed at 0-10cm. At the end of the first crop rotation of the experiment (2012), the treatments which all harvest residues were maintained on the soil without fertilization (ReM-F), all harvest residues were removed from the soil plus fertilization (ReR+F) and harvest residues were partially removed from the soil plus fertilization (ACR+F) presented the highest values for the nutrient.