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Particulate Soil Organic Matter in Bahiagrass-Rhizoma Peanut Mixtures and Their Monocultures

Soil particulate organic matter (POM) represents a labile soil organic matter (SOM) fraction and is an important source of nutrients for soil microorganisms and plants. This study assessed C and N stocks in POM and soil mineral-associated fractions for forage mixtures of rhizoma peanut (RP; Arachis glabrata Benth.) and bahiagrass (Paspalum notatum Flugge) in contrast with their monocultures. Treatments included two bahiagrass entries receiving 90 kg N ha–¹ harvest–¹, two RP entries, and the four bahiagrass–RP combinations. Samples were collected from the 0- to 15-cm soil depth in July 2015 and in September 2016, after seven harvests. Soil samples were fractionated into POM and mineral-associated organic matter (MAOM) by dispersion with sodium hexametaphosphate and sieving (53 μm). Samples were analyzed for δ¹³C, δ¹⁵N, and total C and N. Groups of bahiagrass, bahiagrass–RP, and RP were contrasted. There were no differences in N stocks among forage treatments. Bahiagrass–RP mixtures had greater soil POM-C stock (5.1 Mg C ha–¹) than RP monocultures (4.0 Mg C ha–¹). Soil POM C/N ratio was lower for bahiagrass (18.2) and RP monocultures (16.2) than bahiagrass–RP (20.6). Bahiagrass–RP and bahiagrass had similar soil δ¹³C and were less depleted than RP (–23‰ vs. –26‰ and –26‰ vs. –28‰, for POM and MAOM, respectively). Soil POM-C stock under forage mixtures was not different from N-fertilized bahiagrass, and it was greater than RP. Hence, mixing RP and bahiagrass can be an option to increase POM-C without the need of N fertilizer.