Carbon status and structural stability of soils from differing land use systems in the Kingdom of Tonga

Abstract

Maintenance of soil carbon stocks is vital for the environment at large and for maintenance of soil chemical, physical and biological fertility. Tonga represents a country in agricultural transition from subsistence to commercial production and whilst this is good for the national economy the impact on soil resources is less clear. The major cropped soils, fallow vegetation types and forest systems of Tonga were identified in each island group and samples of representative soils (0.15 m depth) from each land use unit were taken. Total carbon (C T ) and d 13 C were measured and labile carbon (C L ) determined by oxidation with 333 mM KMnO 4 . These data were used to determine the carbon management index (CMI) and the proportion of carbon from C4 species in the C T pool. Relative to primary forest, the soil C T and C L generally declined with changes in vegetation and more intense mechanical tillage. The contribution of C4 plants to soil C increased with intensity of mechanical tillage and the prevalence of C4 guinea grass (Panicum maximum Jacquin) fallow. The changes in soil C were reflected in the CMI, and C L was a more sensitive indicator of change than C T . These data indicates that all land use systems have experienced a large net loss of soil C relative to the forest systems. Soil mean weight diameter (MWD) decreased significantly with increased intensity of mechanical tillage and to a lesser extent with the intensity and length of cropping. The relationship between soil MWD and soil C was similar with soil C T and C L . Grass fallow was as effective as permanent vegetation systems in improving soil MWD and lowering the micro-aggregate (<125 lm) fraction.

Effect of land use change on soil organic carbon

“…It is clear that higher SOC values are in grassland to compare with cropland (Gerzebek et al 2006; Barančíková 2014;Gelaw et al 2014;Manu et al 2014;Sanford 2014). Generally speaking, higher input of plant and root residues in grassland soils is stabilizing the SOC stock in the top soil.…”

Effects of vegetative mulches on growth of indigenous crops in the Kingdom of Tonga

“…() examined the results from 74 experiments conducted in 19 countries and found that mulching increased average yields, water use efficiency and nitrogen (N) use efficiency by up to 60%, and that plastic mulch was more effective than straw mulch. In tropical areas, plastic mulch has not been so successful because of elevated soil temperatures (Manu et al ., ).…”

“…All this intensification has increased pressures on land, and Manu et al . () have shown that intensification resulted in a marked decline in soil carbon (C), wet aggregate stability and nutrient status.…”

Mulch effect on successive crop yields and soil carbon in Tonga

“…Research in Tongatapu Island in Tonga reported by Manu et al . () found that soil where guinea grass mulch had been applied for more than 5 yr had 46% higher total C concentration compared to soil that had been repeatedly mechanically cultivated. Similarly, the mean weight diameter of aggregate was 244% higher and the percentage of aggregates <125 μ m was 78% lower in the soil where guinea grass mulch had been applied.…”

“…Previous research by Manu et al . () has shown that a marked decline in soil C, wet aggregate stability and nutrient status occurred as land use changed from forest to cropping and has suggested that addition of mulches could be a way to arrest this decline in the resource base.…”