Rapid soil C accumulation

Here we report a rapid rate of soil C accumulation accompanying conversion of row crop agriculture land to intensively grazed pastures (Fig. 1) that is on par with the global record for soil C accumulation rate associated with tropical grasslands in Brazil23. Fisher, et al.23 illustrated that the introduction of deep-rooting African grasses to lowland savannas in tropical South America drives C accumulation rates of 7.1 Mg C ha−1 yr−1 and suggested that other fields may have rates as high as ∼13 Mg C ha−1 yr−1 (refs 23, 24, 25). In our pastures, we find that peak C accumulation occurs 2–6 years after pasture establishment with a gain of 8.0±0.85 Mg C ha−1 yr−1 (r2=0.88, P<0.0001) in the upper 30 cm of soil (Fig. 2). Following an apparent lag in the first 2 years, the most recently established farm (converted in 2009) accumulated C at 4.6 Mg C ha−1 yr−1 (r2=0.79, P=0.0184), the middle-established farm (converted in 2008) accumulated C at an average rate of 9.0 Mg C ha−1 yr−1 (r2=0.80, P=0.0395), while the earliest-established farm (converted in 2006) accumulated C at 2.9 Mg C ha−1 yr−1 (r2=0.97, P=0.1125) before an apparent decline in the accumulation rate at 6.5 years following conversion. In all cases, detectable increases in C accumulation were limited to the upper 30 cm (Supplementary Figs 1 and 2), with variability below 30 cm yielding an integrated C accumulation rate of 7.1±2.7 Mg C ha−1 yr−1 in the top metre (Supplementary Fig. 3). This suggests that accumulation at depth may require a longer timeframe or a shift in management practices.

Figure 1: Land use shift from row crop to management-intensive grazing. Aerial photographs of the Wrens Farm (Site ID: 2006) taken in 2006 (a) before conversion to management-intensive grazing system and (b) in 2013, ∼7 years after land conversion. Full size image

Figure 2: Soil carbon rapidly increases with conversion of row crop to intensive grazing. Soil carbon (Mg C ha−1) content shown for the top 30 cm of farms converted in 2006 (green symbols), 2008 (blue symbols) and 2009 (black symbols) and a control farm currently in row crop (grey symbols). Samples from soil pits and soil cores are distinguished by circles and triangles, respectively; open versus closed black circles are from different locations on the 2009 farm. The linear regression (solid line: r2=0.88, P<0.0001) and 95% confidence intervals (dashed lines) are for data between 2 and 6 years since conversion only. The grey-shaded arrows represent our interpretation of soil carbon change in this system on the basis of current data. Full size image

Why we observe high rates of soil C accumulation

Soil δ13C values are an efficient tracer of this land-use change. The 50+ years of C3 cotton and peanut cultivation established soil δ13C values much lower than the major C inputs from grazed pasture grasses, including manure (δ13C=−14.88) and roots (δ13C=−20.03) from the C4 bermudagrass that comprises the bulk of warm season forage on these farms (Supplementary Fig. 4). Consistent with this, the farm exhibiting the highest C accumulation rate also exhibited large changes in δ13C, while the farm established in 2006 showed little to no change in δ13C over the 2-year study period. However, the farm established in 2006 had root mass abundance (standing stocks) two to three orders of magnitude higher than the more recently established farms (Supplementary Table 1), suggesting high belowground C cycling from root turnover.

The transition from crop to pasture systems results in an average 19% increase in soil C stocks26. In our intensively grazed systems, we report an ∼75% increase in C stocks within 6 years of conversion. This high C accumulation rate stems from year round intensive forage/grazing management techniques on sandy soils with an initially low soil C content due to past conventional-till row crop agriculture. The pastures in this study are managed for maximum forage production; employing N fertilization, irrigation and selective rotational grazing designed to optimize forage digestibility and protein content. These forage-management techniques are precisely those suggested to increase SOM in pasture systems13 and when they are applied to soils with degraded SOC content, such as soils in the southeastern United States, rapid C accumulation ensues.

Consequences for soil quality

In addition, the establishment of intensively grazed pastures improved soil quality by increasing soil cation exchange capacity (CEC) and WHC during the 5 year of transition documented by our chronosequence (Fig. 3, Supplementary Figs 5 and 6 and Supplementary Table 2). CEC increased by 95% in the top 30 cm for an average rate of 44 meq g−1 yr−1, while WHC increased by 34%. These soil improvements should reduce the need for fertilizer and water inputs and may also mitigate nitrogen (N) losses from the agricultural system. In a companion study on the oldest chronosequence farm, only half of the 620 kg ha−1 of N inputs (silage, hay, grain and mineral fertilizer) were accounted for as N outputs (milk, N 2 O, NH 3 and leached NO 3 ) with the balance presumably sequestered in the SOM pool or lost via denitrification27.

Figure 3: The relationship between percent soil C and cation exchange capacity. The relationship is shown at different soil depths along the chronosequence. Soil CEC increases with soil C accumulation. The dashed line is a linear regression of the data (P<0.001). Full size image

Six years after conversion, our data suggest that an apparent plateau in SOC accumulation occurs at ca. 38 Mg C ha−1 in the top 30 cm, which is consistent with peak SOC stocks in the region13,20,28. C stocks as high as 51 Mg C ha−1 (top 20 cm) have been measured in a bottomland Piedmont forest soil (ref. 20 and references within), suggesting that C may continue to accumulate in these intensively grazed pastures over the millennial scales at slower rates29. In New Zealand pastures, soil C stocks have been estimated as high as 109 and 138 Mg C ha−1 (refs 14, 17), and once these soils reach a higher SOC level they can become susceptible to C loss if management changes15. Total C stock will be determined by grass productivity, soil physical and biological attributes, and the degree of physical disturbance, which can all change with future management11,20,25.

Extrapolation of results