IV. A. Soil enzyme activities under field conditions
4.2 Acid phosphatase activity during the dry and rainy season
The activity of acid phosphatase under different land uses and season is presented in Table 4.1.a and 4.1.b for the black and red soils, respectively. Acid phosphatase activity was affected by season, land use, and soil type. In the black soils during the dry season, acid phosphatase was 69 mg PNP kg-1 dry soil h-1 in homegardens, 94 mg PNP kg-1 dry soil h-1 in milpas, and 360 mg PNP kg-1 dry soil h-1 in forest. The activities increased during the rainy season, homegardens black soils showed a mean of 96 mg PNP kg-1 dry soil h-1 and milpas black soils presented 196 mg PNP kg-1 dry soil h-1. Forest black soils reached a mean of 1200 mg PNP kg
-1 dry soil h-1 during the rainy season. Red soils showed values lower than the black soils, in the dry season homegardens showed a mean of 56 mg PNP kg-1 dry soil h-1 and 72 mg PNP kg-1 dry soil h-1 in milpas. Forest red soils had a mean of 174 mg PNP kg-1 dry soil h-1. During the rainy season, the acid phosphatase activity also increased in the red soils. Homegardens had a mean of 67 mg PNP kg-1 dry soil h-1 whereas milpas red soils had 44 mg PNP kg-1 dry soil h-1. In contrast forest red soil reached a mean of 579mg PNP kg-1 dry soil h-1.
The acid phosphatase activity was significantly higher during the rainy season than during the dry season (p < 0.0001; Samples-Paired T-Test). The variation in the acid phosphatase activity was significantly related to the moisture content (Table 4.1.c). With the exception of red soil at the milpa system, the activity increased between 16-70 % during the rainy season. This fact could be attributed to the growth period of the plants. In the rainy season the plants require major concentration of nutrients, there is more demand of P and it shoots up the acid phosphatase activity. Since acid phosphatase is considered as adaptive enzyme, the secretion of this enzyme by the plant roots is determined by the P-demand of the plants (Tarafdar and Jungk 1987). In addition, environmental factors have been identified as determinants in the acid phosphatase activity amongst them: moisture was found as a major control of acid phosphatase activity (Rastin et al. 1988, Moscatelli et al. 2001, Boerner et al. 2005). For instance, Sardans et al. (2007) concluded that soil moisture and temperature affect root-surface phosphatase activity. They attributed that if scarce soil water and/or low temperature are limiting plant production, the phosphatase activity also decreases.
IV. Soil Enzymes
Several studies about the seasonal variation on the acid phosphatase have been carried out, mainly in agricultural soils (Deng and Tabatabai 1997, Kraemer and Green 2000, Wick et al.
2002, Gianfreda et al. 2005) but the results have been contrasting. For example, Rastin et al.
(1988), conducted a study of the seasonal variation of various enzymes including acid phosphatase, and reported a lack of a significant relationship between seasonal variation in soil moisture content and acid phosphatase activity. However, acid phosphatase and alkaline phosphatase activities have shown significant variations through the different seasons in semiarid woodland soils. A study reported maximal activities in summer and winter when the temperature reached the maximal and minimal values and the soil moisture varied (Kraemer and Green 2000). Enzyme activity is dependent on many factors, amongst them: aeration, vegetation, microflora, soil moisture, soil temperature, and soil type (Burns 1978), which can produce variation in the activity and originate contrasting results.
Forest soils had the highest activity (577.92 mg PNP kg-1 dry soil h-1) whereas enzyme activities in both milpa and homegardens soils were similar (101.54 and 71.91 mg PNP kg-1 dry soil h-1 in milpa and homegarden respectively). Despite higher organic P concentration in forest and milpa sites, the latter site exhibited lower activity. This might be attributed to more accessible organic P in forest as compared to milpa, where lower moisture conditions could affect the activity. Also, the high activity in forest soils could indicate lower inorganic P conditions, since plant roots secrete acid phosphatase under low availability of P to compensate their deficiency (Tarafdar and Claassen 1988, Tadano et al. 1993). These results are in line with those obtained by Aguila (2007) who reported lower available P in forest (113 mg.kg-1 and 79 mg.kg-1 in the black and red soils respectively) and in milpa soils (142 mg.kg-1 and 88 mg.kg-1 in the black and red soils respectively) than in homegardens soils (739 mg.kg-1 and 593 mg.kg-1 in the black and red soils respectively); while organic P concentration was higher in the forest (21 mg.kg-1 and 23 mg.kg-1 in the black and red soils respectively) and milpa sites (20 mg.kg-1 and 25 mg.kg-1 in the black and red soils respectively) than in homegardens sites (9 mg.kg-1 and 12 mg.kg-1 in the black and red soils respectively).
Pearson’s correlation reported negative and significant correlations of -0.363 between
IV. Soil Enzymes forest sites, where the deficiency of P was reported. However, the activity was not influenced by the organic P concentration. Consequently, it might suggest that the acid phosphatase is influenced by the inorganic P availability, but is constrained by the soil moisture content because this factor is stronger correlated than the available P.
On the other hand, black soils showed higher activity (335.62 mg PNP kg-1 dry soil h-1) than red soils (165.30 mg PNP kg-1 dry soil h-1). High activity in the black soils might be due to sorption of the enzyme onto organic matter. Phosphatase production and activity have been reported to be very sensitive to SOM concentration (Goldstein et al. 1988). Although in the present study it was not determined the SOM content, previous studies have reported that the black soils have higher organic content than the red soils (Weisbach et al. 2002, Shang and Tiessen 2003).
Table 4.1. a. Acid 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 360 ± 75 1200 ± 216
Milpa 94 ±14 196 ± 51
Homegardens 69 ± 31 96 ± 32
FPLSD 111 355
Mean + 1 SD.
Within the same column, differences are significant when greater than FPLSD
Table 4.1. b. Acid 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 174 ± 77 579 ± 137
Milpa 72 ± 14 44 ± 9
Homegardens 56 ± 50 67 ± 29
FPLSD 66 134
Mean + 1 SD.
Within the same column, differences are significant when greater than FPLSD
IV. Soil Enzymes
Factors and Interactions p
Land Use 0.000
Soil Type 0.000
Season 0.000
Land Use x Soil Type 0.000
Land Use x Season 0.000
Soil Type x Season 0.000
Land Use x Soil Type x Season 0.000
Table 4.1. c. Pearson’s correlation coefficients of the acid phosphatase activity (acP) during the dry and rainy season with moisture content, inorganic P and organic P.
acP Dry season acP Rainy season
Moisture 0.680** 0.880**
Inorganic P
(dry 1) -0.363** -
Organic P
(dry 1) 0.108 -
Pearson’s correlation coefficients calculated from means of the determined parameters from all land uses. Inorganic P (Source: Aguila 2007), Organic P (Source: Aguila 2007). N= 90.
** Correlation is significant at the 0.01 level (2-tailed)
1 Dry season field conditions