Effects of juniper cutting on nitrogen mineralization

Western juniper (Juniperus occidentalis spp. occidentalis Hook.) woodlands have rapidly spread into the sagebrush steppe of the northern Great Basin the past 100 years. The influence of juniper on soil N dynamics in cut and uncut woodlands was investigated. Eight 1-ha blocks were established in a juniper woodland. In half of each block all trees were cut with cut trees left on site. Sampling on cut plots was stratified to include intercanopy and debris zones. Debris zones were defined as sites under cut trees which had not previously received inputs of juniper litter. In the uncut treatment, intercanopy and canopy zones were sampled. Sampling was conducted for a 2 year period post-cutting. The first sample year was a moderately dry year and the second sample year was a very wet year. Measured parameters included KCl extractable N (NH4+-N and NO3−-N), nitrification, N mineralization (using buried bag technique), volumetric soil moisture, total soil C and N, and herbaceous biomass. In the dry year, soil moisture, KCl extractable N, and N mineralization were higher in the cut intercanopy zone vs. the other locations. In the wet year, KCl extractable N and N mineralization did not differ among the zones. The effect of year, dry vs. wet, overwhelmed the effect of juniper removal. The initial effect of juniper cutting was an increase in KCl extractable N, but by the second year post-treatment differences for the N variables measured were not apparent. In the dry year there was a higher potential for N loss from soils as a result of the buildup of KCl extractable N. The methods used to assess extractable N and N mineralization indicate that most of the extractable N in soils was taken up by soil micro-organisms and not lost via leaching or denitrification. Soils beneath downed trees had lower nitrification and N mineralization rates compared to the intercanopy. In the uncut woodland, N mineralization and nitrification rates and KCl-extractable NO3− levels in intercanopy soils were greater or equal to rates and levels in the canopy zone. There was not a fertility island effect in canopy influenced soils for available N in the uncut juniper woodland