Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2014
Publication Date: 8/6/2014
Citation: Wuest, S.B. 2014. Seasonal variation in soil organic carbon. Soil Science Society of America Journal. 78:1442-1447.
Interpretive Summary: Accurate quantification of soil organic carbon is important to understanding how soil management can contribute to mitigation of greenhouse gasses. Since changes in soil carbon tend to occur very slowly, it is generally assumed that the timing of soil sampling is not critical. There is very little data on soil carbon measured over seasons, so we conducted an experiment where we sampled monthly for 39 consecutive months. We found that there was a seasonal cycle of 2 to 8%, and as much as 16% variation around the average for the 39 month period. The timing of the seasonal cycle was dependent on soil management. This seasonal variation needs to be accounted for if we are to draw accurate conclusions regarding soil carbon changes over time or between diverse soil management systems.
Technical Abstract: Organic carbon in soil is most often measured at a single point in time, under the assumption that the major pools of organic carbon change so slowly that variation over weeks or months will be insignificant. The validity of this assumption has implications for accurate comparison of soil carbon between sites, treatments, and over time. I took 39 consecutive monthly soil samples from a field experiment planted every year with winter wheat. The total range of variation was 14 to 16% of the average soil carbon content over the 39-month period. From 2 to 8% could be identified as a regular seasonal pattern. The data also indicated differences in seasonal patterns between different soil management treatments. Given the small changes in soil carbon being measured and modeled in many agricultural and natural systems, single point soil samples are likely to introduce substantial measurement error variance.