Effects of contrasting soil fertility on root mass, root growth, root decomposition and soil carbon under a New Zealand perennial ryegrass/white clover pasture

Abstract

The contribution of below ground plant root tissue to soil carbon (C) pools is attracting considerable interest in the context of greenhouse gas mitigation options. A field experiment was conducted on a perennial ryegrass/white clover pasture in the Manawatu, New Zealand, to examine the effect of differing soil nitrogen (N) and phosphorus (P) fertility status on root dynamics. Root standing mass, shoot and root dry matter (DM) accumulation and root tissue decomposition were measured at 6–8 week intervals over one year at moderate (Olsen P = 24, no added N) and high (Olsen P = 49, 400 kgN ha−1y−1 added N) soil fertility levels. Shoot production was significantly greater in the high fertility treatment (2550 cf. 1890 gDM m−2y−1) but differences in root dynamics were confined to two periods in spring and winter. In late spring the pattern was for lower root mass (183 cf. 231 gDM m−2 between 0–80 mm depth) and higher root production (0.71 cf. 0.52 gDM m−2 d−1 between 0–120 mm depth) under higher fertility. In winter the reverse was observed. There is some evidence that the soil type used in the root in-growth cores underestimated root production values for this site by a factor of approx. one third. Short-term differences between the two fertiity treatments in standing root mass and root production did not lead to treatment differences in topsoil C and N changes over four years. This may reflect insufficient separation in the two soil fertility treatments and a low overall root tissue input to soil organic matter.