Recovery of Nitrogen (15N) by Sugarcane from Previous Crop Residues and Urea Fertilisation Under a Minimum Tillage System

Residues of green-harvested sugarcane contribute to nutrient recycling in production systems. Therefore, better understanding of trash decomposition dynamics can help crop fertilisation management. This study was conducted during the 2006–2008 seasons in Jaboticabal, north-eastern Sao Paulo State, Brazil and aimed to evaluate the nitrogen recovery rates from the previous crop residues or from urea applied on sugarcane planting in a minimum tillage system, thus without trash and rhizome incorporation in crop renewal. Previous crop residues consisted of 9 and 3 t/ha of sugarcane trash (dry tops + leaves) and root system (roots + rhizomes) enriched with 1.07 and 0.81% 15N isotope, respectively. These contributed 51 and 33 kg/ha of N. 15N labelled trash laid on the soil surface and buried 15N-root system attempted to simulate the original field residues disposal. The SP81-3250 variety was planted with 80 kg N/ha of a 5.17% 15N-labelled urea. Recovery of sugarcane residues-N (trash-N and root system-N) or urea-N incorporated to the soil at planting were evaluated in distinct plant parts (stem, tops and dry leaves) during three consecutive harvest seasons. Recovery of urea-N was higher in the first harvest season (31% of initial N rate) and its uptake decreased in the second and third to 5 and 4%, respectively. In later harvested seasons, urea-N had probably been turned-over as soil organic matter and/or microbial biomass but remained in the soil N pool and available for plant recovery. Trash-N uptake closely resembled urea-N uptake, and only 13% of its N content was recovered in the first year, followed by 7 and 3% in the second and third seasons. Root system-N recovery was different since the second cut uptake was higher than the first followed by the third, 9, 6 and 2% respectively. Three year cumulative recovery of urea-N, trash-N and root system-N was 39, 23 and 17%, respectively. Most recovered N was found in stems followed by tops and dry leaves.