Microbial biomass nitrogen upon wetting at the end of the incubation experiment

types is summarized in table 3.8.a and 3.8.b. Under ½ FC conditions microbial biomass N showed values around 12.28 – 24.13 mg N kg^-1 dry soil in the black soils and 7.25 – 10.61 mg N kg^-1 dry soil in the red soils. Under FC, microbial biomass N ranged between 8.90 – 21.12 mg N kg^-1 dry soil in the black soils and 3.14 – 4.63 mg N kg^-1 dry soil in the red soils. A statistically significant interaction (p: 0.021) between land use and wetting was detected in the factorial ANOVA test, also paired-samples T-Test reported significant differences between ½ FC (14.06 mg N kg^-1 dry soil) and FC (9.10 mg N kg^-1 dry soil) conditions. Microbial biomass N dropped around 34% under FC in comparison with ½ FC and it was positively correlated with the moisture content under ½ FC conditions (r: 0.660, p: 0.000) and FC conditions (r:

0.475, p: 0.000). Since soils under FC conditions showed higher microbial activity (soil respiration) than soils under ½ FC,therefore this might suppose that FC promote an increase in the microbial activity but no in their biomass. Also, it could be possible that higher soil water content influenced the extractability of soil microbial biomass N in the fumigation process and

III. C-Cycle in Karstic Soils the chloroform did not produce a total biocide effect. In contrast with our results, higher microbial biomass N at 100% FC was observed by Zaman et al. (1999) and Zaman and Chang (2004), which indicated that with the increase of the moisture content there is a rise in the microbial biomass N. However, this study did not find similar results possibly to methodological differences, Zaman and co-workers determined microbial N according to Brooke et al. (1985), whereas in the present study it was determined by ninhydrin-reactive. It is important to stress that the values found in the present experiment are higher than those reported in the dry season but similar than in the rainy season.

Table 3.8. a. Microbial Biomass N by artificial wetting of black soils under different land uses (mg N kg^-1 dry soil).

Land Use ½ FC FC

Forest 24.13 ± 6.19 21.12 ± 9.19

Milpa 20.35 ± 5.54 11.02 ± 3.27

Homegardens 12.28 ± 5.73 8.90 ± 5.62

FPLSD 9.61 10.73

Mean + 1 SD.

Within the same column, differences are significant when greater than FPLSD

Table 3.8. b. Microbial Biomass N by artificial wetting of red soils under different land uses (mg N kg^-1 dry soil).

Land Use ½ FC FC

Forest 7.83 ± 3.04 3.14 ± 2.31

Milpa 10.61 ± 4.23 3.93 ± 2.32

Homegardens 7.25 ± 5.27 4.63 ± 4.93

FPLSD 7.06 5.51

Mean + 1 SD.

Within the same column, differences are significant when greater than FPLSD

Factors and Interactions P

Land Use 0.000

Soil Type 0.000

Moisture 0.000

Land Use x Soil Type 0.000

Land Use x Moisture 0.021

Soil Type x Moisture 0.576

Land Use x Soil Type x Moisture 0.515

III. C-Cycle in Karstic Soils

Table 3.8. c. Pearson’s correlation coefficients of microbial biomass nitrogen upon artificial wetting with moisture content, organic C, microbial activity (CO2 –C) and nitrate.

Nmic ½ FC Nmic FC

Pearson’s correlation coefficients calculated from means of the determined parameters from all land uses. Nmic: microbial biomass N, Corg: organic Carbon (Source: Aguila 2007), CO2 –C: CO2 evolved in the incubation experiment, NO3 –N:

available Nitrate in the incubation experiment (Source: Aguila 2007). N= 90.

** Correlation is significant at the 0.01 level (2-tailed)

1 Dry field conditions

2 under ½ FC condition incubation experiment

3 under FC condition incubation experiment

Land uses influenced the microbial biomass N, forest soils showed the highest microbial N with a mean of 14.30 mg N kg^-1 dry soil. Milpa and homegardens had means of 11.68 and 8.41 mg N kg^-1 dry soil, respectively. Moreover, black soils showed higher microbial biomass N (16.50 mg N kg^-1 dry soil) than red soils (6.46 mg N kg^-1 dry soils). As was also reported in the section III.A (3.5), higher amount of microbial biomass in forest and black soils is attributed to greater availability of substrate for the microorganisms and also to the improve of soil microclimate condition with high litter on surface. Litter serves as substrate that is converted into microbial biomass and SOM (Billore et al. 1995). Litter quantity varies at the land uses; this is higher in forest soils than in milpa and homegardens. High amount of litter on the soil surface improve water infiltration by reducing runoff, maintaining optimal temperature and reducing the evaporation (Spedding et al. 2004). Therefore, better moisture condition and increased SOM can be involved in the positive effect on the microbial biomass in forest soils. The loss of the vegetal material in homegardens soils is reflected in the decline of the microbial biomass; and microorganisms with faster turnover can possibly faster mineralise the maize leaves in milpa. Also, the high fungal biomass in forest system could increase the nutrient immobilization in their biomass, since fungi can maintain more C and N than bacteria. Bacteria have lower C assimilation efficiencies and faster turnover rates than

III. C-Cycle in Karstic Soils fungi (Adu and Oades 1978), therefore this should be an important characteristic in determining the higher microbial biomass in forest system than in homegardens and milpa.

The quantity and quality of litter indirectly affects the microbial biomass N. High organic C concentrations stimulate microbial activities (Gaillard et al. 1999), because organic substrates are sources of energy for the microorganisms (Staafs and Berg 1981, Zaman et al. 1998). In the present test, organic C under dry field conditions significantly correlated with the microbial biomass N under ½ FC and FC (Table 3.8.c). The high organic C concentration found by Aguila (2007) in forest sites and the significant correlation between these parameters suggest that organic C could be used as source by the microorganisms and has influence on the microbial biomass N. However, it is important to highlight that this relation is strongly constrained by the soil moisture, which could indicate that the accumulation of the organic matter in the soil is due to the limited conditions of moisture in Yucatan.

Soil microorganisms are responsible for both production and consumption of inorganic N (Dannenmann et al. 2006). The microbial biomass N under ½ FC and FC conditions was significantly correlated with the CO2 – C evolved and available nitrate under FC and ½ FC (Table 3.8.c). In the present study, microbial biomass N represents both a measure of N retention into active microbial biomass and mineralization of N, because microbial biomass N correlated with both the microbial activity (CO2 – C evolved) and the available nitrate under ½ FC and FC conditions. As it was reported in the previous section (III.B: 3.7.a), the maintenance of microbial activity under optimal moisture conditions indicates that there is enough substrate to decompose and mineralise. Also, increasing microbial biomass is coupled with the soil respiration (CO2 –C evolved) and N mineralization (nitrates). This supports the suggestion that wetting dried soil stimulates the activity of growing decomposer. Indeed it is well established that wetting of dried soils causes an increase in microbial mineralization of organic N (Birch 1958) and organic C (Powlson and Jenkinson 1976).

III. C-Cycle in Karstic Soils

3.9. β -Glucosidase activity upon artificial wetting at the end of the incubation