Uptake of biocrust nitrogen by tree crops in a sandy soil agroecosystem

Abstract

Biological soil crusts (biocrusts) are widespread in dryland ecosystems worldwide and were only recently discovered in agroecosystems. As agroecosystem biocrusts complete biological nitrogen (N) fixation, this research investigated if biocrust N could be released to biocrust subsoil and assimilated by a perennial tree crop. Biocrust was pulse labeled with 15NH4Cl (15N) in the greenhouse for eight weeks prior to application in a citrus (Citrus spp.) orchard in Southwest Florida, USA. Patches of labeled biocrust were applied to twelve citrus trees in September, with twelve trees in the same orchard not receiving biocrust (controls). The 15N in the biocrust, subsoil, crop fibrous roots, and crop leaves was quantified at seven dates leading up to fruit harvest 158 days after application. Sampling dates overlapped with citrus root (September–October) and shoot (March) flush periods. Isotopically labeled biocrust N was present in subsoil and crop roots one day after biocrust application and in leaves after three days. Results revealed that N was released from biocrust at a rate of 0.63% 15N released per day leading up to fruit harvest when 91.7% of 15N had been released. The amount of biocrust 15N in the fibrous roots increased during a root flush period (September–October) until 28 days after biocrust application and then decreased leading up to a leaf flush period and fruit harvest. Approximately 23.8% of total biocrust 15N was in the crop tissue at fruit harvest. Overall, these results indicate agroecosystem biocrusts have the potential to support tree crop growth during N demanding growth stages.