Distinguishing functional pools of soil organic matter based on solubility in hot water

Abstract

Evidence is emerging that the solubility of soil organic matter (SOM) in water is a key factor regulating the turnover of carbon (C) and nitrogen (N). We used data from a field trial with a wide range of treatments in a case study to: (1) examine the link between SOM solubility and bioavailability and (2) evaluate whether low water-solubility is a factor contributing to the persistence of refractory SOM. The trial was established in 2000 on a silt loam (Udic Dystocrept) at Lincoln, New Zealand to identify management practices that maintain SOM following the conversion of long-term pasture to arable cropping. The following land use treatments were sampled (0–7.5, 7.5–15 and 15–25 cm) in 2013: (1) long-term ryegrass-white clover pasture; (2) arable cropping rotation, managed using either intensive, minimum, or no tillage; and (3) continuous bare fallow (plots maintained plant-free using herbicide; not cultivated). The bioavailability of SOM was determined by measuring C and N mineralisation in a 98-day incubation at 25°C (soil maintained near field capacity) and water solubility was assessed by measuring hot-water-extractable C and N (16-h extraction at 80°C). After 13 years of arable cropping, C stocks (to 25 cm) were 11 t ha–1 less than in pasture soil (decrease of 14%). Tillage ‘intensity’ had no effect on C stocks in the top 25 cm. Large losses of C were observed in the bare fallow treatment (19 t C ha–1 less than pasture soil). The bioavailability of SOM (CO2-C mineralised as a proportion of soil C) also declined under arable cropping and bare fallow. The relationship between total C and mineralised C had a significant (P < 0.001) intercept, indicating that part of the organic matter (13 g C kg–1) did not contribute to C mineralisation (it was biologically inert). Across treatments and sampling depths, SOM mineralised in 98 days generally corresponded well with that extracted in hot water. A significant fraction of SOM (~9 g C kg–1) did not release C to hot water. Water-insoluble organic matter, including compounds that are strongly bonded to mineral surfaces, may comprise a significant part of the refractory SOM.