Soil carbon and nitrogen sequestration over an age sequence of Pinus patula plantations in Zimbabwean Eastern Highlands

Abstract

Forests play a major role in regulating the rate of increase of global atmospheric carbon dioxide (CO2) concentrations creating a need to investigate the ability of exotic plantations to sequester atmospheric CO2. This study examined pine plantations located in the Eastern Highlands of Zimbabwe relative to carbon (C) and nitrogen (N) storage along an age series. Samples of stand characteristics, forest floor (L, F and H) and 0–10, 10–30 and 30–60cm soil depth were randomly taken from replicated stands in Pinus patula Schiede & Deppe of 1, 10, 20, 25, and 30years plus two natural forests. Sodium polytungstate (density 1.6gcm−3) was used to isolate organic matter into free light fraction (fLF), occluded light fraction (oLF) and mineral associated heavy fraction (MaHF). In both natural and planted forests, above ground tree biomass was the major ecosystem C pool followed by forest floor’s humus (H) layer in addition to the 45%, 31% and 24% of SOC contributed by the 0–10, 10–30 and 30–60cm soil depths respectively. Stand age caused significant differences in total organic C and N stocks. Carbon and N declined initially soon after establishment but recovered rapidly at 10years, after which it declined following silvicultural operations (thinning and pruning) and recovered again by 25years. Soil C and N stocks were highest in moist forest (18.3kgCm−2 and 0.66kg of Nm−2) and lowest in the miombo (8.5kgm−2 of C and 0.22kg of Nm−2). Average soil C among Pinus stands was 11.4kg of Cm−2, being highest at 10years (13.7 of Ckgm−2) and lowest at 1year (9.9kg of Cm−2). Some inputs of charcoal through bioturbation over the 25year period contributed to stabilisation of soil organic carbon (SOC) and its depth distribution compared to the one year old stands. Nitrogen was highest at 10years (0.85kg of Nm−2) and least at 30years (0.22kg of Nm−2). Carbon and N in density fractions showed the 20year old stand having similar proportions of fLF and oLF while the rest had significantly higher fLF than oLF. The contribution of fLF C, oLF C and MaHF C to SOC was 8–13%, 1–7% and 90–91% respectively. Carbon and N in all fractions decreased with depth. The mineral associated C was significantly affected by stand age whilst the fLF and oLF were not. Conversion of depleted miombo woodlands to pine plantations yield better C gains in the short and long run whilst moist forest provide both carbon and biodiversity. Our results highlight the importance of considering forestry age based C pools in estimating C sink potential over a rotation and the possibility of considering conservation of existing natural forests as part of future REDD+projects.