The effects of burning or mulching of harvest residues on selected soil properties in a Eucalyptus plantation in Northern KwaZulu-Natal.
The sustainable management of commercial forest resources is required to ensure long-term soil fertility and the productivity of later rotations; this includes soil nutrient retention and the protection of soil structure. A major factor in the protection of soil structure is the distribution of soil organic carbon (SOC) and nutrients, as well as the stability of soil aggregates. These are influenced by forest harvest residue management practices such as the removal, displacement on the soil surface, or burning of residues. Soil aggregate stability is an important soil property that gives a reliable estimation of the ability of soils to respond to external forces such as rainfall, wind, and land management. The objectives of the study were to investigate the effect of forestry residue management methods on selected soil properties and their effect on soil structural stability under the previous stump-line and in the current inter-row. The results obtained from the study will contribute to understanding the effects of forest harvest residue management on some soil properties. The study site was near Paulpietersburg, northern KwaZulu-Natal, South Africa. A trial was established by the Institute for Commercial Forestry Research (ICFR) in 2010 to investigate the effect of tree harvest residue management, soil compaction and its amelioration through ripping, on the growth of Eucalyptus dunnii grown on a Magwa soil form. This trial has a factorial treatment design and consists of three soil disturbance treatments (no compaction, compaction, and compaction with amelioration) and two residue management practices (burning and residue mulching), with four replicates (24 treatment plots). The effect of burning and mulching harvest residue on selected soil properties and soil aggregation in relation to other soil properties was studied two years after the treatments were established. For the purposes of the present study, the no-compaction treatment and both residue management treatments (burning and mulching) were used (eight plots). Bulk samples, 192 in total, were collected from each of the eight plots, from both the current inter-row and the old stump-line, at a depth of 0 to 0.1 m and 0.1 to 0.2 m using a spade. Samples were air dried and sieved to collect soil aggregates between 2 and 8.5 mm for soil aggregate stability determinations by determination of the mean weight diameter (MWD). Some of the bulk sample was analysed for SOC, pH, exchangeable bases (Ca2+, Mg2+, K+, and Na+), and particle size distribution. Statistical analyses were carried out using Genstat and the results were regarded as significant if p < 0.05 (or 5%). Residue management had no significant (p ≥ 0.05) effect on exchangeable Mg2+, K+, and Na+, soil pH, exchangeable acidity, effective cation exchange capacity and texture. There was, however, a significant effect of residue management on SOC and exchangeable Ca2 Ca2+. Under the mulched plot treatment, SOC at the 0 to 0.1 m depth was significantly higher in soils when compared to the burned plot treatment. Under the burned plot treatment, exchangeable Ca2+ concentrations were significantly higher than the mulched treatment. Furthermore, under the burned plot treatment; exchangeable Ca2+ and Mg2+ concentrations were significantly higher in the 0 to 0.1 m depth than at the 0.1 to 0.2 m depth. Neither residue management strategy had any significant effect on MWD. However, there was a significant effect on MWD relating to soil depth and sampling position. Under both treatments, the MWD was higher at 0 to 0.1 m than at the 0.1 to 0.2 m depth, in both inter-row and stump-line samples. The average MWD of 2.45 mm (with a standard error of 0.04) and values that ranged between 0.8 and 3.5 mm was indicative of stable aggregates. Thus, surprisingly, no significant correlation was found between SOC and MWD. However, it was concluded that changes in residue management may alter SOC, thus impacting on the productivity of the soil. Furthermore, it was suggested that soil aggregate stability was driven by exchangeable Ca2+ and Mg2+.