Soil C and N sequestration and fertility development under land recently converted from plantation forest to pastoral farming

Abstract

Soil organic matter accumulation and concomitant fertility changes in soils recently converted from plantation forest to pastoral agriculture in the Taupo-Rotorua Volcanic Zone have been observed, with a probable soil C sequestration rate of 6.1 t ha-1 year-1 , and a soil N sequestration rate of 0.451 ha-1 year-1 , to 150 mm soil depth, for the first 5 years after conversion attwo of three selected farms. Rapid increases in Olsen P were observed, with soils reaching their optimum agronomic range within 3-5 years after conversion, at two of three farms. A decreasing C:N ratio with time since conversion reflects improved fertility status, and implies that in initial years of pasture establishment, N losses are reduced due to its immobilisation into soil organic matter. These research findings suggest that land-use change from plantation forest to pastoral farm, with inputs of N, P, K and S to soils, allows significant soil C and N sequestration for at least 5 years after conversion. This rate of C sequestration could be used as an offset for forest C sink loss in future emissions trading systems. Further research is required to at least 0.3 m depth to confirm this preliminary study.

Changes in soil total C and N contents at three chronosequences after conversion from plantation pine forest to dairy pasture on a New Zealand Pumice soil

“…New Zealand studies on forest and pasture comparisons on other Soil Orders have also reported greater total C under pastures than under forest or scrub, but those studies generally sampled to much shallower soil depths (Walker et al 1959;Giddens et al 1997;Hedley et al 2009;Schipper and Sparling 2011;Hewitt et al 2012) than used in the current study. There is often greater variability in total C contents of lower soil horizons (Franzluebbers 2009;Kravchenko and Robertson 2011), which can mask changes in the total soil profile, even when changes in the A horizon may be significant (Kravchenko and Robertson 2011;Schipper and Sparling 2011).…”

“…However, the rates of accumulation in our study were slower than some previously reported. Hedley et al (2009) suggested that overall accumulation of total C on Taupo Pumice Soils for the first 5 years after conversion was equivalent to 0.61 kg C m -2 year -1 . In our study, accumulation rates of soil C in the whole profile over the first 10 years after conversion were 0.2 kg C m -1 year -1 at Wairakei and 0.21 kg C m -1 year -1 at Atiamuri, with no significant change at Tokoroa.…”

“…However, most of these studies were single paired comparisons of mature pine forests and established permanent pastures (Giddens et al 1997;Davis and Condron 2002) and only sampled to 7.5 or 15 cm (Giddens et al 1997;Sparling and Schipper 2004;Hedley et al 2009). Single sample comparisons do not allow rates and patterns of change to be calculated.…”

“…Davis and Condron (2002) suggested that where sites were converted from pasture to forest (the reverse process), at least 10 years would be required before any substantial change in soil C stock would be detected. Because of the considerable soil disturbance caused by removing trees, windrowing and recontouring, Hedley et al (2009) recommended that soil sampling of converted pastures should be to at least 30 cm. Hewitt et al (2012) used sampling to 30 cm depth for paired-site comparisons of radiata pine plantations and low productivity grassland, but Blanco-Canqui and Lal (2008) recommended sampling to 60 cm depth for comparison of tillage methods to conserve soil C. A complication in sampling to greater depth is that soils from the B and C horizons contain much less C, and often show greater variability in C content (Franzluebbers 2009) and relatively small change in C and N concentrations (Poeplau et al 2011).…”

“…Single sample comparisons do not allow rates and patterns of change to be calculated. Hedley et al (2009) used a chronosequence of pasture sites, but the sequences were only 1-5 years after conversion from plantation forestry to pasture, and there were no long-term forestry sites against which to compare the converted pasture. Davis and Condron (2002) suggested that where sites were converted from pasture to forest (the reverse process), at least 10 years would be required before any substantial change in soil C stock would be detected.…”