Browsing by Author "Nieto Lawrence, Juan Sebastian"

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    The effects of forest residue management on topsoil nutrient dynamics, and on early growth responses of Eucalyptus grandis x urophylla under varied levels of fertilization
    (Stellenbosch : Stellenbosch University, 2022-03) Nieto Lawrence, Juan Sebastian; Du Toit, Ben; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: The sheer mass of forest residues, or slash left behind after harvest hampers site accessibility and poses a significant fire hazard. A low intensity burn is normally sufficient to reduce the fuel load, but other methods of management that retain residues may be more sustainable practices regarding soil nutrient capital. In this study, different methods of slash management were investigated on two sites in KwaZulu-Natal, measuring early effects on growth of Eucalyptus spp. and soil nutrient dynamics. These slash management practices included residue retention (R; control), mulching (M), burning (B), and burn-and-disking (DB). Applied in factorial to these were four levels of fertilization: a low dose of N-rich fertilizer (LowN), a high dose of N-rich fertilizer (HighN), a low dose of P-rich fertilizer (HighP), and an unfertilized control (O). On the first site, in Zululand, KwaMbonambi, E. grandis x urophylla was planted on deep sands, and large responses to slash management were measured within a year of planting. The B and DB treatments resulted in less acidic soil pH values, and a pulse of plant available P, Ca2+, and Mg2+, with no detectable effect on soil mineral N.This pulse caused young eucalypts planted on B and DB treatments to growsignificantly taller and wider than those on M and R treatments, being between 0.4 m and 0.7 m taller at six months after planting, and between 1.5 cm and 2.0 cm wider at breast height by eleven months after planting. Growth on DB treatments was significantly greater than even that on B treatments, with no response to fertilization on either treatment. Eucalypts on M and R treatments, however, displayed a sensitivity to fertilizer treatments, with growth improved most by HighP fertilization and least by the unfertilized control. This highlighted an interaction between residue management and fertilization. The M treatment, specifically, resulted in the greatest response to fertilization, the poorest overall growth of eucalypts, and an early locking-up of soil N expressed as soil C/N ratios greater than 30, and net immobilization after aerobic incubation of soil samples. The R treatment produced marginally better growth than the M treatment, as well as an early pulse of N mineralization that had disappeared within six months. Interestingly, the mineralization potential of soils from all treatments decreased steadily over time, showing net mineralization initially, but net immobilization by the end of the study period. This indicated a decreasing substrate quality over time. On the second site, in the Midlands, Hilton, E. grandis x nitens was planted on humic topsoils with apedal subsoils. Similar responses to the B and DB treatments were expressed in the soil within half a year of planting, but a pulse of NH4+was also found to be associated with these treatments. The M and R treatments, conversely, resulted in a pulse of NO3−, expressed most significantly five months afterplanting. These results highlighted short-term site-specific responses to residue management, as well as early influences of two relatively new methods of slash management: mulching, and disking after a burn.