Accumulation of soil organic C and change in C:N ratio after establishment of pastures on reverted scrubland in New Zealand

Abstract

Rates of organic carbon accumulation and changes in C:N ratio are reported for 10 New Zealand soils converted to pastures from scrub. The data were derived from archive papers originally published in 1964, but which did not report on changes in the C contents of the soils. The soils had been sampled to 0–7.5, 7.5–15, and 15–30 cm depths and chronosequences of up to 66 years obtained by selecting sites with differing times since pasture establishment. We calculated changes in the mass of C and N in the 0–7.5 cm depth and compared that to the mass in the 0–30 cm depth of soil. The shortest time over which organic matter change was assessed was 18 years and the longest was 66 years. Nine of the ten soils showed increases in the C contents of the 0–7.5 cm depth soil, and a natural logarithmic curve generally gave a better fit to the time course data than a linear fit. However, when the full 0–30 cm depth was considered, only two soils showed a significant increase in total C, changes in the C contents of other soils were non-significant, and two soils showed a decline in total C. The rates of change in the C contents were averaged over 0–5 years, 5–25 years and 25–50 years. Across all 10 soils, the mean rates of accumulation of C in the 0–7.5 cm depth were 1.07 (between 0 and 5 years), 0.27 (between 5 and 25 years) and 0.09 Mg C ha−1 year−1 (between 25 and 50 years) and significantly (P < 0.05) greater than zero. Very similar rates were obtained for the 0–30 cm depth of soil with mean rates across all soils 1.01 (0–5 years), 0.25 (5–25 years) and 0.09 Mg C ha−1 year −1 (25–50 years), respectively. In the 0–7.5 cm depth of soil, total Kjeldahl N (TKN) increased significantly in seven of the 10 soils. When expressed for the 0–30 cm depth of soil, only five soils still showed significant increases in TKN contents over time. Using the data for the 0–7.5 cm depth, the predicted time (mean and standard error) for the soils to reach a C:N ratio of <10 was 46 ± 17 years. The soils were originally sampled over 44 years ago, suggesting that currently (2009), very few of them could be expected to have capacity for further N storage in organic matter in the surface soil unless there was an increase in soil C. Changes in soil C and N in the shallow upper soil layers are easily masked by the relatively small changes in C and N contents and much greater masses of soil at lower depths.