Labile organic matter as an indicator of organic matter quality in arable and pastoral soils in New Zealand

Labile organic matter fractions (light fraction C, microbial biomass C and water-soluble organic matter) were extracted from two soils (Lismore silt loam and Temuka clay loam) varying in cropping history from long-term (>9 yr) arable cropping to long-term (>9 yr) pasture in the Canterbury region of New Zealand. With increasing periods under pasture, soil organic C content increased and the amounts of labile organic matter extracted increased (microbial biomass C, 1.0–2.2% of organic C; light fraction, 1.8–4.6% organic C and water-soluble C, 0.7–1.2% organic C). Labile organic matter was more closely correlated with previous cropping history (R=0.89–0.96) than with soil organic C content (R=0.81–0.82). Alternating these soils under rotations of 2–5 yr of pasture followed by 2–5 yr arable resulted in soil organic C remaining unchanged while labile organic matter increased under pasture and declined under the arable phase. The three indices of labile organic matter were closely correlated suggesting they are interrelated properties. In the Lismore soil the mean proportion of total soil organic C, N and P present in water-soluble form differed widely being 0.28, 0.18 and 0.03% respectively for field-moist samples. This presumably reflects differences in chemical nature, solubility, biodegradability and affinity for soil colloids of soil organic C, N and P compounds. Water-soluble organic C, N and P was much greater when extracted from air-dried than field-moist soils and this difference was proportionately greater for soils with higher total soil organic matter contents. Water-soluble organic matter in air-dried soils was thought to have originated from soil solution, from lysed desiccated microbial cells and from labile humic material. It was concluded that inclusion of grazed pastures in a cropping system maintains labile organic C in higher amounts than is possible under annual cropping.