Effect of repeated soil heating at different temperatures on microbial activity in two burned soils.

The effect of fire severity and recurrence on the recovery of enzymatic activities (ß-glucosidase, urease, acid phosphatase) and bacterial activity was monitored. Unburned and burned soil samples from soil affected by a high severity wildfire and by a low severity experimental fire were subjected in laboratory to a temperature gradient to simulate different fire severities. These samples were subjected to a second laboratory heat treatment to simulate the effect of recurrence. Soil temperature was measured and used to calculate the degree-hours reached by the soil. The results showed: a) a strong effect of repeated soil heating at different temperatures on soil microbial activity; b) a different sensitivity of enzymatic activities and bacterial activity to fire, c) the magnitude of changes in these biochemical properties was related to the extent of heat supplied to samples and the previous fire/heat history, and d) degree-hours are adequate to quantify the severity of heat treatments and to examine their effects on soil microbial activity. The relationships between degree-hours and the different biochemical properties analyzed clearly demonstrate that the usefulness of these biochemical properties to detect the soil microbial community response to the heat stress followed the order: urease activity > acid phosphatase activity > ß-glucosidase activity ≫ bacterial activity.

Soil CO2 emissions from Northern Andean páramo ecosystems: effects of fallow agriculture.

The effects of fallow agriculture on soil organic matter (SOM) dynamics and CO2 emissions were assessed in the tropical Andean páramo ecosystem. Possible changes during the cultivation-fallow cycle were monitored in four areas of the Quebrada Piñuelas valley (Venezuela). Uncultivated soils and plots at different stages of a complete cultivation--fallow cycle were incubated, and SOM mineralization kinetics was determined. Soils exhibited a low SOM mineralization activity, total CO2 evolved never reaching 3% of soil carbon, pointing to a stabilized SOM. Potential soil CO2 effluxes differed significantly according to their plot aspect: northeast (NE)-aspect soils presented higher CO2 effluxes than southwest (SW)-aspect soils. Soil CO2 emissions decreased after ploughing as compared to virgin páramo; low CO2 effluxes were still observed during cropping periods, increasing progressively to reach the highest values after 4-5 y of fallow. In all cases, experimental C mineralization data was fitted to a double exponential kinetic model. High soil labile C pool variability was observed, and two different trends were identified: NE-oriented soils showed more labile C and a wider range of values than SW-facing soils. Labile C positively correlated with CO2 effluxes and negatively with its instantaneous mineralization rate. The instantaneous mineralization rate of the recalcitrant C pool positively correlated with %C evolved as CO2 and negatively with soil C and Al2O3 contents, suggesting the importance of aluminum on SOM stability. The CO2 effluxes from these ecosystems, as well as the proportion of soil C released to the atmosphere, seem to depend not only on the size of the labile C pool but also on the accessibility of the more stabilized SOM. Therefore, fallow agriculture produces moderate changes in SOM quality and temporarily alters the CO2 emission capacity of these soils.

Atrazine degradation and enzyme activities in an agricultural soil under two tillage systems.

The content of atrazine and its metabolites (hydroxyatrazine, deethylatrazine and deisopropylatrazine) as well as the activities of two soil enzymes (urease and beta-glucosidase) were evaluated in an acid agricultural soil, located in a temperate humid zone (Galicia, NW Spain), with an annual ryegrass-maize rotation under conventional tillage (CT) and no tillage (NT). Samples were collected during two consecutive years from the arable layer at two depths (0-5 cm and 5-20 cm) and different times after atrazine application. Hydroxyatrazine and deisopropylatrazine were the main metabolites resulting from atrazine degradation in the acid soil studied, the highest levels being detected in the surface layer of the NT treatment. A residual effect of atrazine was observed since hydroxyatrazine was detected in the arable layer (0-5 cm, 5-20 cm) even one year after the herbicide application. Soil enzyme activities in the upper 5 cm layer under NT were consistently higher than those in the same layer under CT. Urease and beta-glucosidase activities decreased with depth in the profile under NT but they did not show any differences between the two depths for the plots under CT. For both tillage systems enzyme activities also reflected temporal changes during the maize cultivation; however, no consistent effect of the herbicide application was observed.