The Impact of Trash Management on Soil Carbon and Nitrogen: I Modelling Long-term Experimental Results in the South African Sugar Industry

Trash blankets contain substantial amounts of nitrogen (N) and other nutrients. The availability of N in trash is complicated because most of the N cycles through the soil organic matter. To gain insights into the long-term fate of N contained in trash, N dynamics in the SASEX long-term trash management experiment (BT1) were simulated with the APSIM-Sugarcane cropping systems model. Initially, the model was verified against cane yields and soil organic carbon data from the experiment. Then, simulations were conducted over 60 yr (1940 to 2000) with N fertiliser application rates ranging from 20 to 260 kg/ha. Cane yields and soil organic carbon results from the BT1 experiment were well represented by the model. The N response simulations showed that cane production not only responds positively to trash retention at the BT1 site, but that the response is dependent on the amount of N fertiliser applied to the crops. To maximise the production benefits from trash retention, additional N (approximately 60 kg/ha) should be applied where trash is retained compared to that where trash is burnt. The additional N is required for the crops to reach their potential with the additional moisture available in the trash retained system. The simulations also show that it takes three to four decades for the response of cane yields to N fertiliser application rates to stabilise, although even after this time soil organic C is still simulated to be declining (as occurred in the BT1 experiment). This study illustrates the additional insights that can be gained into the complexities of agricultural systems with the aid of a comprehensive simulation tool. Analyses of the likely impacts of trash retention at other sites in the South African sugar industry will be reported in a companion paper.