Organic orchard floor management impact on soil quality indicators: nutrient fluxes, microbial biomass and activity

Maintaining nutrient availability in organic orchards presents challenges. Diverse litter sources may increase nutrient cycling and the efficiency by which microbes utilize carbon (C). Two tree-row treatments: ‘straw-mulch’ (Triticum aestivum L.) and ‘living-mulch’ (Lobularia maritima (L.) Desv.), and two alleyway groundcovers: ‘grass’ (Festuca rubra with Lolium perenne L.) and a legume, ‘Birdsfoot trefoil’ (Lotus corniculatus L.) were compared to an industry standard, tillage with a grass alleyway. Tree-row deposited trefoil biomass contributed 0.2 kg additional total nitrogen (N) per tree annually. Soil from tree-rows with trefoil alleyways had 23% greater organic C (+ 3.1 g kg−1), 17% greater total N (+ 0.3 g kg−1), up to 53% greater microbial biomass (+ 204 mg CO2–C kg−1), 32, 34 and 31% greater dehydrogenase (+ 2.3 µg TPF g−1), alkaline (+ 55.9 µg p-nitrophenol g−1) and acid (+ 106.7 µg p-nitrophenol g−1) phosphomonoesterase enzyme activity, 62% greater soil NO3−N (+ 2.05 µg NO3−1 g−1), and 51% higher nitrification rates (+ 0.22 µg NO2−–N + NO3−–N g−1 soil h−1) than tree-row soils adjacent to grass alleyways. Straw-mulch and living-mulch soils did not differ. Metabolic CO2 quotient (qCO2) values were lowest in trefoil (0.0032) and living-mulch with grass treatments (0.0036), indicating greater microbial growth efficiency. Tillage and straw-mulch with grass alleyway treatments had the highest qCO2 (0.0053 and 0.0048) and the lowest microbial biomass (246.3 and 297.6 mg CO2–C kg−1 soil). Higher potential biochemical activity, and total C and N, suggests orchards with trefoil alleyways cut and deposited into tree-rows, enhances soil organic matter and promotes balanced nutrient cycling and retentive processes.