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

Abstract

Core Ideas Bahiagrass–rhizoma peanut mixtures resulted in similar whole soil C stocks as N‐fertilized bahiagrass monocultures. Bahiagrass–rhizoma peanut mixtures had greater total and particulate organic matter C than rhizoma peanut monocultures. Particulate organic matter C/N ratio of rhizoma peanut and N‐fertilized bahiagrass monocultures were lower than C/N ratio in bahiagrass–rhizoma peanut mixtures. The use of δ13C to verify changes in the soil organic matter caused by the plant component was a better predictor than the use of δ15N. 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 Flüggé) 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.