Nitrogen mineralization in semi-arid rangeland : influence of grazing and land use type

Mineralization of nitrogen (N) in soil is an important process that controls life on earth. In this study, the monthly pattern of soil mineralization was assessed by using intact soil incubation cores in the upper soil layer (0-15cm). The experiment was conducted in semi-arid rangeland of Tomagh, Balochistan, Pakistan in three land use types: protected, orchard and grazed. The study was conducted from May 2009 to April 2010. Results showed that there was highly significant (P<0.01) response in N-mineralization, N-ammonification, and N-nitrification both on monthly and land used basis. The N-mineralization, N-ammonification and N-nitrification were higher during growing season in the months where there was higher rainfall. Maximum mineralization (0.7602 μg g), ammonification (0.7482 μg g) and nitrification (0.4879 μg g) occurred in orchard area, whereas minimum for the same attributes were recorded in grazed area (0.0291, 0.0200 and 0.0091 μg g respectively). In conclusion, the temperature and moisture of soil were found to be controlling factors for N-cycling. Moreover, grazing in Tomagh semi-arid rangeland exerted significant negative influence on N-cycling.


Introduction
While considering the materialistic substance, it can be said that the most abundant elements on earth is Nitrogen (N).For the life to exist 2 to 20 per 100 carbon atoms are present in different life form [1].In soil the available N exists in different organic forms of nitrogen, like nitrate (NO3 -) and ammonium (NH4 + ).The range of N quantity present in vegetation is from 1-240 g N/m 2 [2].The N mineralization in soil plays an important role in its bioavailability of N to plants and soil microorganisms in many ecosystems [3, 4].It influences soil fertility [5, 6], plant growth [7] and primary production [8].This is the biological process of conversion of organic N to inorganic forms i.e. nitrate (NO3 -) and ammonium (NH4 + ).N mineralization is influenced by many factors such as plant and microbial  The experimental area was divided in three types; grazed (unprotected), ungrazed (protected) and orchard (of apple tree).The temperature of soil was recorded with digital thermometer from all three sites (Table 1).A pair of PVC tubes, 15 cm long and 3.5 cm in diameter, were penetrated to a range 10 cm in soil in all three experimental sites in first week from May 2009 to -April 2010 for incubation to assess nitrogen mineralization.
The initial inorganic nitrogen (NH4 + -N + NO3 --N) analysis was done by taking out one tube immediately from the soil and the second tube was left for incubation on the site.All the tubes were covered on the top by a film, having eight 1mm diameter holes of plastic piece.Tubes were collected after incubation period of 30 days for inorganic N analysis of soil within tubes.After removal of stones and coarse roots by hand sorting, from soil cores, thoroughly hand mixing of soil was done.After sieving with a 2 mm mesh size soil was stored at 4ºC. were selected randomly (3-Replicates; 1 replication = shared sample of three replicates of either mixed species).At each collection location a distance of 1 to 10 m among sampled flora was observed.Every month repeated all the above said procedure in plots of grazed, ungrazed and orchard.N mineralization on a dry mass basis, throughout growing season, was defined as the change before and after incubation in concentrations of inorganic Nitrogen, net N ammonification was measured being difference among NH4 + -N concentrations pre and post of incubation, Whereas the net N nitrification was measured being difference among NO3 --N concentrations pre and post incubation process.For all the incubation periods net amount of mineralized and nitrified N was summed for calculating total annum net mineralization and mineralization of nitrogen [25, 26].

Results
Results showed that there was a significant difference in temperature between different months and land type (P<0.05).In general, Soil temperature was mostly lower while soil moisture was mostly higher in orchard as compared to the other two sites, while soil moisture was higher in protected than in grazed area (Table 1).ANOVA showed significant difference in mineralization, net ammonification and net nitrification between sites and months (P<0.05,Table 2).The mineralization of N, ammonification and nitrification were higher in orchards than grazed area while protected area had intermediate values for these N contents (P < 0.05; Tables 3 to 5).The optimum soil pH for ammonification ranges between 6.0 to 8.0 and for nitrification and between 7.5 to 8.0 [35].The pH of our study area was alkaline i.e. average 7.88 (pH values ranged from 6.9 to 8.0%).Higher rate of ammonification in our case study as compare to nitrification is unknown.While concluding, greater nitrogen mineralization was analyzed in orchards that possessed a decreased soil temperature and increased soil moisture versus the rest of sites.It is further registered through this research that grazing, soil temperature and moisture are important controlling factors of mineralization of nitrogen [36].

Conclusion
Nitrogen cycling may be controlled by the soil temperature and soil moisture in Tomagh Semi-arid rangelands.Besides, substantial adverse impact on N-cycling has occurred because of over grazing in area.

Figure 1 .
Figure 1.Monthly rainfall during 2008, 2009 and average of over previous eight years biomass, moisture and temperature of soil [9].In the soil sulfur, N and phosphorous are present more than 90% in organic form [2].

Table 1 . Mean for net N-ammonification (µg g -1 ) of the studied lands soil during the months of June 2009 to April, 2010
cNo major difference P < 0.05 in the mean values preceding same letters, although grand mean values of row, preceding same letters also showing No major difference P < 0.05.

Table 3 . ANOVA (factorial) run on N-mineralization, net ammonification rates and net nitrification (µg g -1 ) of soil in all three selected plots Source of variation Mineralization rate Ammonification rate Nitrification rate
NS = Non-significant at 5% LSD Level.*= Significant at 5% LSD Level.** = Significant at 1% LSD Level

Table 4 . Mean for net mineralization (µg g -1 ) of the studied lands soil during the months of June 2009 to April, 2010
No major difference P < 0.05 in the mean values preceding same letters, although grand mean values of row, preceding same letters also showing No major difference P < 0.05

Table 5 . Nitrification (µg g -1 ) of the studied lands soil during the months of June 2009 to
No major difference P < 0.05 in the mean values preceding same letters, although grand mean values of row, preceding same letters also showing No major difference P < 0.05 included in this study, but their effects on N mineralization is ensured.Grazing has positive effects as decrease in SOC and MB in addition to decreasing infiltration of air and water because amplified compaction [