Microbial biomass nitrogen during the dry and rainy season

The quantity of the microbial biomass N as affected by land use, season, and soil type is summarized in Tables 3.4.a and 3.4.b. During the dry season the microbial biomass N ranged between 2.47 – 13.57 mg N kg^-1 dry soil in the black soils and 2.01 – 3.87 mg N kg^-1 dry soil in the red soils. In the rainy season, the values varied between 4.81 – 24.46 µg N kg^-1 dry soil and 2.79 – 7.50 mg N kg^-1 dry soil in the black and red soils respectively. The black soils

III. C-Cycle in Karstic Soils showed higher microbial biomass (13.11 mg N kg^-1 dry soil) N than the red soils (5.21mg N kg^-1 dry soil). Also, microbial biomass N was significantly higher during the rainy season (8.91 mg N kg^-1 dry soil) than the dry season (4.93 mg N kg^-1 dry soil) and was affected by the interactions land use x soil type; land use x season; and soil type x season. In the present study, microbial biomass N was in the same range in Chichihuan desert soils reported by Gallardo and Schlesinger (1995), who found values around 15 and 10-17 (mg N. kg-1 soil).

However, these values are lower than those reported by Srivastava (1992), Hossain et al.

(1995) and Li and Chen (2004) who found values around 39 –100 mg N. kg^-1 soil in different agricultural management, soil type and texture.

The microbial biomass N increased about 45% in the rain compared with the dry season.

Correlation analysis showed that microbial biomass N was significantly related with the moisture content during the dry season (p< 0.0001; r=0.47) and during the rainy season (p<

0.0001, r=0.74). High values in the microbial N during the rainy season are related with an increase in the moisture and nutrient availability. Under optimal moisture conditions there is high nutrient availability that the microorganisms can immobilize in their biomass. Also, the low values in the dry season could be that under low soil moisture many microorganisms are intolerant to the drought conditions (Reid 1980, Harris 1981, Piao et al. 2000). Previous studies (Ross 1987, Wardle and Parkinson 1990, Van Gestel et al. 1993) suggested that soil drying-rewetting cycles enhance the turnover of the microbial biomass. During the dry season part of the biomass is killed and on the rewetting in the rainy season, the surviving biomass can utilize the cell debris (Kieft et al. 1987); thus microbial activity may have been higher during the rainy season than during the dry period when the moisture level are low. Normally seasonal fluctuation in biomass N follows different trends; many authors have found that abrupt changes in soil moisture stimulate the turnover of microbial biomass in soils (Wardle and Parkinson 1990, Mazzarino et al. 1991, Gallardo Schlesinger 1995, Piao et al. 2000).

However, others have shown little soil microbial biomass variation throughout the seasons (Bardgett et al. 1999, Spedding et al. 2004, Tonon et al. 2005). This variation can be explained by variation in the climatic conditions of temperate and tropical regions, differences in texture, and type of management, amongst others.

III. C-Cycle in Karstic Soils

Table 3.4. a. Microbial biomass N (mg N kg^-1 dry soil) in the black soils under different land uses during the dry and rainy season.

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

Table 3.4. b. Microbial biomass N (mg N kg^-1 dry soil) in the red soils under different land uses during the dry and rainy season.

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

Factors and Interactions P

Results from this study show that the land uses have had significant effects on the microbial biomass N. Means by land uses indicate that microbial biomass N varied among land uses and were approximately 57 % higher in forest than in milpas and 68 % higher in forest than in homegardens soils. Several studies conducted in the tropics have indicated that the conversion of forests into several agricultural practices is often accompanied by a significant reduction in the SOM, nutrients (especially N and P), and microbial biomass (Prasad et al. 1994, Templer et al. 2005). Also, it is well known that changes in the microbial biomass pools coincide with changes in the availability of mineralizable substrate or soil conditions (Spedding et al. 2004).

It is important to stress that N is poorly immobilized by the microbial biomass, this part of the total N represent about 0.06 % in black forest soils, whereas in milpa and homegardens

III. C-Cycle in Karstic Soils microbial biomass immobilized around 0.03% of the total N. Low microbial biomass N in the milpa and homegarden soils could be due to a low availability of mineralizable substrate and to soil conditions that create a different microclimate and promote changes in the microbial biomass. Under shifting cultivation (milpa) lower input of organic matter is provided;

similarly regular removal and burning of organic material in homegardens let to reduced organic matter input. This observation is in agreement with Benjamin et al. (2001), who reported low contribution of nutrient from the litter in homegardens of Yucatan due to this is removed. They stated that if litter were not removed, potential N contribution in homegardens of Yucatan would be very high. However, in the forest the high input of organic matter that stays on the soil is an important source for microbial growth and, allows an increase in the microbial biomass. In addition, the composition of plants is dominated by legumes, which often have lager amounts of litterfall N, thus the large input of this material could also be a further explanation of high microbial N in the forest. Our observation is based on a study carried out by Ruiz-Garvia (unpublished data 2007) who reported high amount of N in the litter and leaf of forests in Yucatan. Ruiz-Garvia reported values around 17 mg N. g ^–1 litter and 26 mg N. g^-1 leaf of forests at different ages in Yucatan. About the differences reported between soil types, the higher biomass N in the black soils can be attributed to the high organic matter content in the black soils, which creates major substrate availability to mineralise and immobilize in the microbial biomass. This observation can be stated by the significant correlation between the organic C and the microbial biomass in the dry season (r:

0.639, p< 0.0001).