IV. A. Soil enzyme activities under field conditions
4.3 Alkaline phosphatase activity during the dry and rainy season
The activities of alkaline phosphatase are presented in Table 4.2.a and 4.2.b for black and red soils, respectively under different land uses and seasons. In the black soils during the dry season, alkaline phosphatase was 160 mg PNP kg-1 dry soil h-1 in the homegardens, 189 mg PNP kg-1 dry soil h-1 in milpas and 331 mg PNP kg-1 dry soil h-1 in forest soils. Red soils showed values of 99 mg PNP kg-1 dry soil h-1 in homegardens, 134 mg PNP kg-1 dry soil h-1 milpas and 176 mg PNP kg-1 dry soil h-1 in forest soils. During the rainy season the activity increased, the black soils presented activities of 263 mg PNP kg-1 dry soil h-1 in homegardens, 128 mg PNP kg-1 dry soil h-1 in milpas and 293 mg PNP kg-1 dry soil h-1 in forests. Red soils showed activities of 159 mg PNP kg-1 dry soil h-1 in homegardens, 124 mg PNP kg-1 dry soil h-1 in milpas and 130 mg PNP kg-1 dry soil h-1 in forest. Alkaline phosphatase was determined by the interactions land use x season and land use x soil types.
IV. Soil Enzymes No significant differences were found between rainy season and dry season. Stabilization of the enzyme by the organic matter could be the cause of the no variation in the activity between the seasons. Wick et al. (2002) reported similar results and they proposed that despite the influence of environmental changes, the enzyme activity might remain stable. In agreement with the finding, Haynes (1987), and Ross et al. (1995), did not find variations in the alkaline phosphatase activity with changes in the seasons. However, Kraemer and Green (2000) found significant differences during the changes of season. They reported peaks of activity during the winter and summer in semiarid woodland.
Alkaline phosphatase had the highest activity in the forest sites (232.43mg PNP kg-1 dry soil h
-1) compared to the milpa (143.84 mg PNP kg-1 dry soil h-1) and homegardens sites (170.23 mg PNP kg-1 dry soil h-1), also differences between red and black soils were found. The activity of alP in milpa and homegarden was 39% and 26% lower, respectively compared to forest. The black soils had higher activity (227.30 mg PNP kg-1 dry soil h-1) than the red soils (137.03 mg PNP kg-1 dry soil h-1). Because alkaline phosphatase is produced by microorganisms, the higher activity in both, forest and black soils, indicate greater biological activity in these soils.
Also, the higher activity in forest and black soils is due to differences in the organic matter decomposition in the different soil types (black and red) and ecosystems. These results can be related to those reported in the microbial activity, where forest soils showed the highest respiration (Chapter III, Section II.B. (3.7)). It is important to highlight that negative and significant correlation was reported by the alkaline phosphatase activity and the CO2 –C evolved under dry field condition. However, under controlled and optimal conditions of moisture (½ FC and FC) significant and positive correlation between the CO2 –C evolved and alkaline phosphatase was reported (Table 4.2.c). These results indicate that under optimal moisture conditions, microbial activity increases and therefore alkaline phosphatase is produced. High microbial activity is desirable in decomposing the plant residue deposited on the soil surface so the nutrients contained in the residue can be recycled (Lindenmann et al.
1984). In the forest system, there is a major accumulation of the organic material that can be used as substrate for the microorganisms, which produce alkaline phosphatase. Alkaline phosphatase is produced by soil microorganisms and soil fauna (Chhonkar and Tarafdar 1984, Nakas et al. 1987), but principally by soil fungi (Dick and Tabatabai 1984, Tarafdar and Claassen 1988). A significant and positive correlation of the alkaline phosphatase with the
IV. Soil Enzymes fungi activity (CO2 –C under Str treatment) (Table 4.2.c) was found; this result is coupled with those reported in chapter III (section 3.7.a, b), which show higher activity of fungi under Str inhibition in forest soils. In addition, high density of hyphae in the forest soils during the samplings was observed. Tarafdar et al. (1989) reported high alkaline phosphatase activity and significant fungal communities under trees and grass for arid soils in India.
Table 4.2. a. Alkaline phosphatase activity (mg PNP kg-1 dry soil h-1) in black soils under different land uses during the dry and rainy season.
Land Use Dry season Rainy season
Forest 331 ± 100 293 ± 71
Milpa 189 ± 19 128 ± 42
Homegardens 160 ± 39 263 ± 41
FPLSD 127 160
Mean + 1 SD.
Within the same column, differences are significant when greater than FPLSD
Table 4.2. b. Alkaline phosphatase activity (mg PNP kg-1 dry soil h-1) in red soils under different land uses during the dry and rainy season.
Land Use Dry season Rainy season
Forest 176 ± 55 130 ± 25
Milpa 134 ± 35 124 ± 29
Homegardens 99 ± 64 159 ± 57
FPLSD 87 65
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
Season 0.914
Land Use x Soil Type 0.000
Land Use x Season 0.000
Soil Type x Season 1.000
Land Use x Soil Type x Season 0.178
IV. Soil Enzymes
Table 4.2. c. Pearson’s correlation coefficients of alkaline phosphatase activity (alP) during the dry and rainy season with moisture content and microbial activity (CO2 –C).
alP Dry season alP Rainy season
Moisture 0.732** 0.432**
Pearson’s correlation coefficients calculated from means of the determined parameters from all land uses. CO2 –C: CO2 evolved in the incubation experiment. N= 90.
* Correlation is significant at the 0.05 level (2-tailed)
** Correlation is significant at the 0.01 level (2-tailed)
1 Dry season field conditions
2 under ½ FC condition incubation experiment
3 under FC condition incubation experiment
4 under ½ FC condition under bacteria inhibition with Str Str: Streptomycin