Soil carbon is greater under grazed paddocks than fencelines, likely due to greater dung returns

The conversion of New Zealand’s native forests to pastoral farmlands increased soil C stocks by about 13 C ha −1 to 1 m depth, but the mechanism(s) behind this increase are unclear. One possible explanation is that increased C returns through grazing and dung deposition increase soil C stocks. To assess the importance of dung inputs for soil C stocks in pastures, we collected soil samples from 38 paired paddock and fenceline sites on six Waikato farms (five grazed by dairy cows and one grazed by beef cattle). Mapping of 2000 dung pats confirmed that only one pat was within the fenceline sampling zone, and so the fenceline sites acted as a control without dung inputs. Average Olsen P values (0–0.1 m) were 11.7 µg g −1 higher ( P < .001) in paddocks than under fencelines, supporting differential nutrient inputs from excreta. Soil samples were analysed in 0.1 m depth increments to 0.6 m for C and N stocks (456 samples from 38 sites and 6 depths/site). Soil C and N stocks were 10.3 C ha −1 and 0.96 N ha −1 higher in the paddock samples than under fencelines ( P < .05; 0–0.6 m) on an equivalent mass basis. Significant differences in C and N stocks were observed deeper in the soil profile suggesting that contributions from dung inputs were not limited to surface soils. Physical fractionation of the 0–0.1 m samples revealed that the increase in C stocks at the top depth was due to greater particulate organic matter C (POM-C) in paddock samples, while mineral-associated C (MAOM-C) content was not significantly different relative to fenceline samples. We propose that continuous dung return elevates POM-C and that dung inputs was a likely driver of greater total C under grazed paddocks. As dung, and likely POM-C, is more rapidly degraded, the higher total soil C in the paddock would likely be vulnerable to rapid C loss if dung returns decreased. • We quantified soil C and N stocks under fencelines and in adjacent grazed paddocks. • Total soil C was 10.3 t ha −1 higher in paddocks than under fencelines (0–0.6 m). • Total soil N was 0.96 t ha −1 higher in paddocks than under fencelines (0–0.6 m). • Dung deposition only occurred in paddocks increasing soil C. • Soil C increase was primarily associated with particulate organic matter (0–0.1 m).