Estimated Nitrogen Balance of Young Apple Trees Grown In Response to Various Organic Apple Production Systems

Annual nitrogen (N) balance can be very useful information for farmers to estimate N loss and surplus after the growing season, especially at the farms where a large amount of N input is applied. A certified organic apple orchard was established to study the interaction of ground cover management systems and nutrient sources on orchard performance in the southern region of the US in 2006. Trees received one of four ground cover management treatments as an under canopy mulch: 1) urban green compost (GC); 2) refuse wood chips (WC); 3) shredded commercial paper (SP); and 4) mow-and-blow vegetation (MB). Across all ground covers, one of three nutrient source treatments was applied: A) a commercial pelletized; poultry-based product (CF); B) composted poultry litter (PL); C) control – the ground cover treatment provides nutrients (NF). GC and WC treated trees had significantly higher total leaf nitrogen (N) content and leaf nitrogen use efficiency (LNUE) which was strongly correlated with trunk cross sectional area (TCSA) in the third year. The estimated annual N surplus was ranged 23 to 1453 g N per tree. GC with any fertilizers had the highest N surplus and [NO₃-N] in soil solution at 30 cm depth in mid-September. Therefore, GC treated tree had more risk for N leaching or immobilization during non growing season when the trees are not actively taking up N. In order to observe which production systems could be more efficient for tree growth and reducing N loss, three dimensional graphs for the TCSA, LNUE, and N surplus showed that WC treated trees had the highest TCSA and LNUE and comparably low N surplus than GC treated trees. GC treated trees had the lowest photosynthetic nitrogen use efficiency during July, August, and September.