How do nitrogen fertilization and cover crop influence soil C-N stocks and subsequent yields of sugarcane?

Abstract

Sugarcane production for biorenewables demands substantial inputs of nitrogen (N) fertilizer that can lead to environmental consequences, and no clear effect has been reported on soil organic carbon (SOC) and N stocks. The use of a previous legume crops cultivation to add N to soil is a promising strategy to reduce the dependence in synthetic N fertilizer, having an international appeal for sustainable production of bioenergy crops. This study investigates a four-year impact of N fertilization on SOC and N stocks and sugarcane ratoon yields in a sandy and clayey soils, while considering the effects of a legume cover crop cultivated during the renovation period on decreasing N fertilizer requirements. Treatments with N fertilizer rates were arranged in a randomized block design after plant-cane harvest: 60, 120, 180 kg N ha−1 and control (without N) in two paired areas previously managed with Crotalaria spectabilis and bare fallow. Soil samples for SOC and N stocks were quantified at the beginning of the trial, and measured again along with microbiological attributes after four consecutive years, while sugarcane yield was assessed annually. SOC and N stocks were unaffected by N fertilizer rates, whereas the cultivation of cover crop increased soil N storage and microbial biomass carbon. There was limited responsiveness of sugarcane ratoons to additional N supplied by inorganic fertilizer under cover crop, whereas positive responses occurred under bare fallow. Considering the average of four-harvest yields, cover crop increased sugarcane yield by 9 % (5 Mg ha−1) at sandy soil and 15 % (16 Mg ha−1) at clayey soil relative to bare fallow. Cover crop provided an annual N fertilizer replacement of 9 and 15 kg ha−1 in the sandy and clayey soils. The long-lasting historical effect of cover crop in increasing sugarcane yield per area without the need for extra N from fertilizer opens a new possibility to increase the sustainability and alleviate associated greenhouse gas emissions of sugarcane fields by allowing savings in synthetic N fertilizer.