Seasonal change and nitrogen cycling in a patchy Negev Desert: a review

Nitrogen cycling in the Negev Desert is concentrated in brief periods of intense microbial activity following rainfall events. In the Negev, plant litter accumulates under shrub patches during the dry period (May-October). In addition, nitrogen originating from dust deposition and snail feces, accumulates. The wetting period may be divided into two subperiods, one at the beginning of the rainy season (November-January) and the second at the end of the rainy season and beginning of spring (February-April). The accumulation of fresh plant litter in the shrub patches may be critical for preserving nitrogen for the beginning of the rainy season (34.4 g N m(-2) a(-1)). This includes the amount of nitrogen input that is intercepted by the shrub patch within the overland water runoff (0.15 g N m(-2) a(-1)), wind borne dust that is deposited on the biological soil crusts (0.09 g Nm(-2) a(-1)), N-fixation (4.32 g N m(-2) a(-1)), and annual plant litter (0.73 g N m(-2) a(-1)). The loss is by denitrification processes (2 g N m(-2) a(-1)) and ammonia volatilization (0.1 g N m(-2) a(-1)). Nitrogen that is associated with accumulated plant litter may be released as ammonia and nitrates by microbial decomposition in the spring, at the last part of the rainy season. This is ideal for optimization of the efficiency of nitrogen cycling in relation to the it-hole ecological system, since there is rapid plant growth in the Negev Desert, from February to mid April. The amount of snail feces found in the shrub patches was 20% of the total organic matter. This has been found to be the major component of the nitrogen budget (38.4 g m(-2) a(-1)). In this manner the crust patches foster the increase in productivity and diversity of the shrub patches and assist them in becoming islands of fertility.