High variability of symbiotic nitrogen fixation in farming conditions

Symbiotic nitrogen fixation (SNF) in perennial legume-grass leys is the primary source of N to organic farming systems in northern countries. This work aimed to quantify SNF in organically managed red clover (Trifolium pratense L.)-grass leys and to relate SNF to explanatory variables. The study was carried out on 27 farms located in the coastal regions of Finland and included 117 ley crops subjected to regular organic farming practices. SNF was calculated from clover herbage dry-matter yield with an empirical model. In the accumulated first and second cuts of one- and two-year-old red clover-grass leys, SNF averaged 185.4 kg N ha-1 yr-1 (SD ± 90.0 kg N ha-1 yr-1); fixation in the aftermath added 62.1 kg N ha-1 yr-1 (SD ± 49.8 kg N ha-1 yr-1). Due to the poor persistence of red clover, SNF declined with ley age. Between- and within-field coefficients of variation of SNF in one- and two-year-old leys averaged 51.1 and 51.8%, respectively. SNF was positively related to soil fertility parameters, mainly to soil structure. It is concluded that the preceding crop value of legume-grass leys needs to be assessed individually. The spatial heterogeneity of soil properties can be reduced through site-specific amelioration and regular applications of animal manure