Potential nitrification rates of semiarid cropland soils from the Central Ebro Valley, Spain

The nitrification capacity of agricultural soils has received little attention in semi-arid Mediterranean environments in spite of the importance of the NO3- in the ecosystem. In this study, a laboratory experiment was carried out to evaluate potential nitrification in six agricultural soils from semiarid Central Ebro Valley (northeastern Spain). Triplicate topsoil samples (0-15 cm) were collected seasonally over a 2-year period. The nitrification capacity was evaluated by fresh soil incubation at 25 degrees C and field capacity with a NH4+-N source. Verhulst's equation was used to express the accumulation of NO3--N with time. The potential nitrification rate (K-max) was derived from the equation and used to quantitatively characterize the nitrification process. The K-max(,) obtained by fitting a sigmoidal curve, allowed us to distinguish the K-max, nitrification capacity of each semiarid agricultural soil. The varied between K-max seasons which indicate that time-variable soil properties (such as salinity and climate) influenced the nitrification rate even in an incubation experiment. On an annual average, K-max, for saline or highly gypsiferous soils ranged from 8.5 to 9.4 mg NO3--N kg(-1) d(-1). For the rest of the soils K-max ranged from 36.3 to 43.7 mg NO3--N kg(-1) d(-1). Potential nitrification rate was negatively correlated (P < 0.01) with soil salinity and positively correlated with total organic C, microbial activity, and microbial biomass. Highly gypsiferous soil showed a low K-max (8.9 mg NO3--N kg(-1) soil d(-1)) with the lowest NO3--N content in field samplings (7 kg NO3--N ha(-1) as an annual average). This fact demonstrated certain inhibition of the nitrification that can be attributed to unbalanced properties in the highly gypsiferous soil.