Are Soil Mineralizable Nitrogen Pools Replenished during the Growing Season in Agricultural Soils?

Seasonal changes in both environmental conditions and substrate quality and quantity determine the soil N supply to plants. The objective of this study was to evaluate seasonal changes in the quantity and quality of soil mineral N (SMN) and mineralizable N pools under a growing potato (Solanum tuberosum L.) crop that was preceded by three crop species (barley [Hordeum vulgare L.], Italian ryegrass [Lolium multiflorum Lam.], and red clover [Trifolium pratense L.]) that might be expected to affect soil N supply. Seasonal changes were small and expressed mainly through changes in SMN and Pool I, a labile mineralizable N pool, which decreased significantly from May to August and then subsequently increased in October. In comparison, potentially mineralizable N (N-0) did not change during the growing season. The depth distribution of mineralizable N pools was also studied, and these were similar in size for the 0- to 15- and 15- to 30-cm layers but decreased for 30 to 45 cm. The estimated depletion of SMN and Pool I for the 0- to 30-cm layer from May to August was less than the supply of N to the potato crop, estimated as plant N uptake (N accumulation in tubers plus vines in late August). This suggests that mineralizable N pools are actively replenished during the growing season, which contradicts the use of a first-order equation in field prediction studies that assume N-0 is depleted with time. These results highlight the need for better understanding of what controls the size and kinetics of the mineralizable N pools to accurately predict N mineralization during the growing season.