IV. B. Soil biochemical characteristics upon wetting and laboratory incubation
4.6. Alkaline Phosphatase activity upon artificial wetting
The ranges in the alkaline phosphatase activities are shown in Table 4.5.a and 4.5.b. In black soils, the activities vary from 160-326 and 120-211 mg PNP kg-1 dry soil h-1, under ½ FC and FC conditions respectively. The red soils reported lower values than the black soils, within ranges of 116-159 and 76-107 mg PNP kg-1 dry soil h-1 under ½ FC and FC respectively.
Under ½ FC conditions, the activity in the homegardens black soils was 160 mg PNP kg-1 dry soil h-1. In milpa, black soils reported activities of 165 mg PNP kg-1 dry soil h-1, whereas forest black soils presented the highest activity with a mean of 326 mg PNP kg-1 dry soil h-1. Red soils showed values of 116 mg PNP kg-1 dry soil h-1 in homegardens, 119 mg PNP kg-1 dry soil h-1 in milpas and 159 mg PNP kg-1 dry soil h-1 in forest soils. Under FC conditions, alkaline phosphatase activity decreased, black soils had activities of 120 mg PNP kg-1 dry soil h-1 in milpa, 126 mg PNP kg-1 dry soil h-1 in homegardens and 211 mg PNP kg-1 dry soil h-1 in forests. Red soils presented activities of 76 mg PNP kg-1 dry soil h-1 in milpas, 78 mg PNP kg-1 dry soil h-1 in homegardens and 107 mg PNP kg-1 dry soil h-1 in forest. During the wetting experiment, alkaline phosphatase was determined by the interaction land use x soil types.
Alkaline phosphatase activity was affected by the moisture effect under laboratory conditions.
Paired-Samples T-Test analysis showed significant differences between ½ FC (174 mg PNP kg-1 dry soil h-1) and FC (119 mg PNP kg-1 dry soil h-1). During the incubation experiment, positive correlation coefficients of 0.573 and 0.484 under ½ FC and FC, respectively, were found between the alkaline phosphatase activity and moisture content (Table 4.5.c). In contrast to the results obtained in section 4.3, the artificial moisturizing influenced the activity of alkaline phosphatase. In this case, the variation of the alkaline activity might be attributed to prolonged incubation time under controlled conditions of moisture and temperature as well as to the states of the enzyme. Studies about changes of the alkaline phosphatase with moisture have been widely reported (Kraemer and Green 2000, Wick et al. 2002, Sardans et al. 2006);
in general, these studies did not find changes in the activity with changes of the soil moisture.
This is attributed to the stabilization of the enzyme by fresh organic matter in the soil fraction that is protected against decomposition (Busto and Perez-Mateos 1995). In contrast to the present test, these measurements have been carried out under moisture field conditions.
IV. Soil Enzymes Forest soils showed the highest alkaline phosphatase activity (201 mg PNP kg-1 dry soil h-1), milpa (120 mg PNP kg-1 dry soil h-1) and homegardens (120 mg PNP kg-1 dry soil h-1) systems reported similar activities. Also, alkaline phosphatase activity showed differences between the different soil types. Black soils had higher activity (185 mg PNP kg-1 dry soil h-1) than red soils (109 mg PNP kg-1 dry soil h-1). Similar to the results found in section 4.6, forest sites and black soils showed the highest activity, confirming the higher biological activity in these soils.
The high leave input on the forest soils and the high organic matter content in black soils promotes high microbial activity and consequently the production of alkaline phosphatase is increased.
A strong and positive correlation of alkaline phosphatase activity with the organic C under dry field condition, CO2 – C evolved under FC and ½ FC conditions and CO2 – C evolved with bacteria inhibition was reported (Table 4.5.c). As is expected, the alkaline phosphatase activity had a strong correlation with both, organic C and soil respiration as measure of microbial activity. Also, with bacteria inhibition (Str) the alkaline phosphatase was correlated, emphasizing the importance of the microbial activity and fungi as producers of the alkaline phosphatase (Dick and Tabatabai 1984, Tarafdar and Claassen 1988) and highly effective in the mineralization of organic P (Kucey 1983 in Wick et al. 2002). Studies carried out by Deng and Tabatabai (1997) and Wick et al. (2002) have also stated the correlation of alkaline phosphatase with soil organic C and microbial biomass. Additionally, alkaline phosphatase only under ½ FC conditions correlated with the organic P under dry field conditions (Table 4.5.c). No correlation under FC conditions was found. It is important to highlight that in Yucatecan soils, the predominant fraction of organic P is NaOH-Po (Aguila 2007), which is considered a resistant organic fraction (Agbenin and Tiessen 1994). The quality of the organic matter has a strong influence on phosphatase activity. In general, phosphatases are better associated with fresh organic matter in the larger soil fractions than with stable and humified organic matter (Rojo et al. 1990). Then, it is possible that these factors play an important role on the alkaline phosphatase activity and their production was strongly mediated by the microorganisms (higher correlation coefficient) than for the organic and inorganic P fractions.
IV. Soil Enzymes
Table 4.5. a. Alkaline phosphatase activity by artificial wetting of black soils under different land uses (mg PNP kg-1 dry soil h-1).
Land Use ½ FC FC
Forest 326 ± 112 211 ± 87
Milpa 165 ± 46 120 ± 29
Homegardens 160 ± 89 126 ± 48
FPLSD 143 99
Mean + 1 SD.
Within the same column, differences are significant when greater than FPLSD
Table 4.5. b. Alkaline phosphatase activity by artificial wetting of red soils under different land uses (mg PNP kg-1 dry soil h-1).
Land Use ½ FC FC
Forest 159 ± 50 107 ± 42
Milpa 119 ± 27 76 ± 29
Homegardens 116 ± 62 78 ± 39
FPLSD 80 61
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 0.071
Soil Type x Moisture 0.270
Land Use x Soil Type x Moisture 0.243
IV. Soil Enzymes
Table 4.5. c. Pearson’s correlation coefficients of alkaline phosphatase activity (alP) after artificial wetting with determined parameters.
Pearson’s correlation coefficients calculated from means of the alkaline phosphatase under ½ FC and FC condition with determined parameters from all land uses. CO2 –C: CO2 evolved in the incubation experiment, Corg: organic Carbon (Source:
Aguila 2007), Total Po: Total organic P (Po-HCO3 + Po-OH + Po-HClc) (Source: Aguila 2007). N= 90.
** Correlation is significant at the 0.01 level (2-tailed) alP: alkaline phosphatase
Str: Streptomycin
1 Dry field conditions
2 under ½ FC condition incubation experiment
3 under FC condition incubation experiment
4 under ½ FC condition and inhibition with Streptomycin