Evaluation of organic matter storage in aggregate of clayey soils under degraded pasture and cropland in central Zagros

Soil carbohydrate, particulate organic carbon and distribution of organic matter in different aggregate fractions are factors affecting soil structure. This study was initiated in a clayey soil in Javanmardi plain (East Lordegan, central Zagros) to evaluate 1) the influence of conversion of degraded pasture to cropland (wheat fields) on total organic carbon (TOC), particulate organic carbon (POC), total nitrogen (Nt), dilute acid extractable carbohydrate (CHda), hot water extractable carbohydrate (CHhw), and 2) the amount of TOC, Nt and CHda inside of aggregates. Soils of degraded pasture and cropland were sampled in two depths of 0-5 and 5-15 cm. The results showed that carbohydrates and POC represent 3.6-6.5% and 39-52% of TOC in soils, respectively. Conversion of degraded pasture to cropland resulted in an increase in TOC, POC, Nt, CHda, and CHhw contents. For the degraded pasture, the amount of TOC, N and CHda were significantly different in aggregate size classes, and appeared to increase as size increased from 0.05 to 1 mm diameter. Nonetheless, there was no clear relationship between the size of aggregates and organic matter content for cropland. The POC/TOC ratio was higher in cropland compared to the degraded pasture, reflecting the coarser nature of organic inputs under cultivation. In general, results indicated that the OC contents in the macroaggregates were higher than those in the microaggregates fractions. This may resemble that the factors controlling the stability or destruction of macroaggegates can control the OC stocks in soils too.