Soil enzymatic activity and wheat grain yield under cover crop systems

ABSTRACT For years, the impacts of using cover crops in productive systems have been measured by their relation with soil chemical and physical characteristics. Consequently, the effects on the soil microbiological characteristics have been little explored. This research aimed to measure the short-term effects of cover crop systems on the enzymatic activity of arylsulfatase and beta-glycosidase, as well as the wheat grain yield. Thirty-five cover crop systems (18 single and 17 intercropped) were implemented, with 3 replications of the following variables for each treatment: enzymatic activity of arylsulfatase and beta-glycosidase, soil organic matter and sulfur contents, and wheat grain yield. The data were submitted to descriptive analysis, multivariate cluster analysis by dendrograms for the single and intercropped plant systems, and t-test for independent samples between the average scores of each group in the dendrograms. Independently of the crop system, there were short-term effects on the enzymatic activity and grain yield. Plants from the same botanic family presented different effects among them. Therefore, in the short-term, cover crops affect the enzymatic activity, and plants that present a higher enzymatic activity do not necessarily result in higher grain yields.

In the no-tillage system, one of the main practices is crop rotation with cover crops, which allows an increase in organic matter, improvements in physical properties, decrease in erosion, suppression of weeds and enhancement in soil microbiological characteristics, besides promoting the nutrients cycling, consequently increasing the soil quality in general (Nunes et al. 2011, Skora Neto & Campos 2017).
Many studies on the benefits of cover crops are directed to soil chemical and physical properties (Bressan et al. 2013, Salomão et al. 2020, Bertolino et al. 2021), being necessary more researches on the dynamics between cover crops and soil biological quality.The main indicators used worldwide analyze the activities of microorganisms in the soil through basal respiration or result from the activities of certain enzymes, with this last method presenting a high sensitivity to detect earlier alterations, in addition to present a higher correlation to nutrients cycling (Silveira 2007).
Enzymes are biomolecules that have an important participation in the cycles of elements in the soil, having a great contribution for the stability of ecosystems (Martens et al. 1992).Such is the case of the arylsulfatase enzyme, which acts directly in providing sulfur for plants, and the betaglycosidase enzyme, which acts in the final stage of cellulose decomposition, being really important for the carbon cycle (Tabatabai 1994).Since it is related to microorganisms, flora and fauna, the enzymatic activity has a great potential to be used as a soil quality indicator, once it is sensitive to variations induced by management (Revoredo 2005).
Therefore, the hypothesis of this research is that the use of cover crop systems may influence directly on the enzymatic activity of arylsulfatase and beta-glycosidase in the soil and wheat grain yield.Thus, this study aimed to evaluate the shortterm effects of cover crop systems on the enzymatic activity of arylsulfatase and beta-glycosidase, as well as on the wheat grain yield.
According to the Köppen classification, the region is Cfa, with humid subtropical climate, presenting an average rainfall rate of 1,881 mm and average air temperature of 19.1 ºC (Alvares et al. 2014), with well-defined summer and winter seasons and without a defined dry season (Silva Filho et al. 2021).Figure 1 presents the meteorological data during the experiment.
The soil collection was carried out at the depth of 0-0.15 m, on May 27, with three replications for each treatment, totaling 105 samples.To make each sample, eight subsamples were prepared, in which the soil was catalogued and sent to the laboratory for chemical determination of sulfur and organic matter contents, as well as to measure the enzymatic activities of arylsulfatase and beta-glycosidase (adapted from Tabatabai 1994), determined in μg p-nitrophenol g -1 soil h -1 .On June 28, the sowing of the wheat TBIO Audaz cultivar was performed with a mechanical seeder, in an average density of 330 seeds m -² and spacing of 0.17 m.The base fertilization used 200 kg ha -1 of the commercial formula 07-40-00 (N-P 2 O 5 -K 2 O).Cover fertilizations were carried out with 150 kg ha -1 of KCl and 100 kg ha -1 of urea, when the wheat was in the tillering phenological stage.On the stretch phenological stage, 100 kg ha -1 of YaraBella, commercial formula 27-00-00 (N-P 2 O 5 -K 2 O), were applied (CQFS-RS/SC 2016).
The wheat crop procedures were the same in all the treatments.The wheat was mechanically harvested on Nov. 11, and data obtained with sensors that measured the grain yield (Shiratsuchi 2004).At the harvest final stage, a file with georeferenced grain yield was generated, processed by the SMS Advance ® Ag Leader Tecnology software, and exported to the CR -Campeiro 7 software (Giotto & Robaina 2007).Then, spatialized wheat grain yield data for each cover crop system were obtained, with values adjusted for humidity of 13 %, and three wheat grain yield values for each treatment.
The data were submitted to an exploratory analysis (descriptive statistics), intending to verify their position and dispersion.Arylsulfatase, beta- glycosidase, organic matter, sulfur in the soil and wheat grain yield values were used in a grouping multivariate statistical analysis by dendrograms, with a complete linkage, and the Euclidean distance was used as a measure of dissimilarity (Callegaro & Longhi 2013).
In order to find treatments with similar answers in the evaluated variables, the cover crop systems were previously subdivided into two: single and intercropped.Based on this, the isolated grouping analysis for each system was performed.After that, the average scores were calculated for each variable, in each group, and compared two by two with the Student t-test (p < 0.05) for independent samples.The statistical analysis was carried out by the R software version 3.2.2(R Core Team 2018) and the Bioestat 5.0 software (Ayres et al. 2007).

RESULTS AND DISCUSSION
The results regarding the descriptive analysis of soil features are presented in Table 1.In relation to the coefficient of variation (CV), for Warrick & Nielsen (1980), the classification of the soil features is divided into low (CV < 12 %), moderate (12 % < CV < 60 %) and high (CV > 60 %) variability.Therefore, the data presented a low variability for organic matter, moderate variability for both enzymes and a high variability for sulfur contents in the soil.The coefficient of variation for the wheat grain yield was only 13.7 %, being considered a moderate variation.
For the multivariate dendrograms analysis performed only with single systems, the cover crops were subdivided into four groups (Figure 2), in which the group S1 is formed by different crop species, like legumes (bean, white lupin, Crotalaria spectabilis and common vetch) and grasses (corn and Coracana eleusine); group S2 by grasses (white forage sorghum, white oat and black oat), legumes (pingeon pea, Crotalaria juncea and hairy vetch), a cruciferous one (fodder radish) and a polygonaceae (buckwheat); group S3 only by grasses (millet and common oat); and group S4 only by legumes (gray velvet bean and jack bean).
The groups presented medium values for the variables (Table 2).It is possible to observe that the S1 and S2 groups differed from each other only due to the enzymatic activities, presenting organic matter and sulfur contents in the soil and wheat grain yield without a significative difference.So, despite this variation in the enzymatic activity, there was no relation to wheat grain yield, when comparing the groups S1 and S2.e-ISSN 1983-4063 -www.agro.ufg.br/pat-Pesq.Agropec.Trop., Goiânia, v. 53, e73792, 2023 Simon et. al (2017), evaluating cover crops in succession to the corn crop, observed that, for the enzymatic activity of arylsulfatase, the crops of the Fabaceae family (legumes) showed lower activity values than for the cover crops of the Poaceae family (grasses), except for the millet crop, which did not differ from the legumes in the study, and, in turn, does not corroborate the data found for the groups S3 (158.8 μg p-nitrophenol g -1 soil h -1 ) and S4 (71.1 μg p-nitrophenol g -1 soil h -1 ) from single crops, where the arylsulfatase activity in the only legume group did not differ from the only grass group.On the other hand, in a study developed by Balota et al. (2004), in a no-tillage system, the enzymatic activity of arylsulfatase presented higher levels in the grass rotation (corn/wheat) at the layers of 0-0.05 m and 0.10-0.20 m than in the ones with legume and grass rotation (soy/wheat) at the same layers.
The wheat grain yield in the groups showed a better result (4,672.0kg ha -1 ) for single legumes (S4) and the lowest one (3,246.4kg ha -1 ) for single grasses (S3) (Table 2).The lower grain yield in groups with grasses is explained by the high C/N ratio of grasses in general, where, due to the succession of crops (corn/grass/wheat), the immobilization of mineral N by organisms in the soil is higher for straw decomposition (Aita & Giacomini 2006).
In table 3 are presented the average scores of variables for each intercropped group, in which, similarly to the single crop systems, no significant difference was observed between organic matter and sulfur levels in the soil, showing that they do not have a direct influence on the variables.
In the same way, wheat grain yield is influenced by several factors in the production system, some of them related to previous cover crops.biomass production in different crop systems may be directly related to wheat plantability.Chen et al. (2007) noticed a reduction in the number of emerged plants after systems with more mass production.
In the present research, as observed by Sá (1993), it was identified that, in successive grass crops, it is necessary more N supplementation, due to the high C/N ratio that causes nitrogen immobilization by microorganisms for decomposition of plant material (Calonego et al. 2012).Because legumes have the ability to fix atmospheric N through symbiosis, in addition to their low C/N ratio, decomposition becomes faster, thus releasing nutrients to the successor crop (Ceretta et al. 1994).Therefore, the wheat grain yield in single systems with legumes was 1,425.6 kg ha -1 higher than in single systems with grasses.CONCLUSIONS 1.In the short term, cover crops affect the enzymatic activity of arylsulfatase and beta-glycosidase; 2. Cover crops have a significant effect on wheat grain yield; 3. Plants from the same botanical family impact differently the enzymatic activity and wheat grain yield; 4. It was possible to group cropping systems with similar response in the enzymatic activity of the different enzymes and in the wheat grain yield; 5.The groups of plants that show a higher enzymatic activity do not necessarily result in higher wheat grain yield.

Figure 1 .
Figure 1.Rainfall, minimum and maximum daily temperatures, from February to December 2021, in Cruz Alta, Rio Grande do Sul state, Brazil.Source: (Brasil 2022).

Table 1 .
Descriptive analysis of soil features and wheat grain yield.

Table 2 .
Average levels for each group of single systems.

Table 3 .
Average levels for each group of intercropped systems.Soil enzymatic activity and wheat grain yield under cover crop systems when studying Helianthus annuus L., Crotalaria juncea L., Raphanus sativus L., Pisum sativum L. and Fagopyrum esculentum Moench, noticed that sunflower as cover crop resulted in higher levels for wheat grain yield, while the other cover crops showed no difference, when compared to the control treatment.Despite that, some researches show differences for wheat grain yield when using different cover crops, even in fertilization associated to N. For example, in a study analyzing crotalaria and hairy vetch under different N doses, Nunes et al. (2011) verified that the use of Crotalaria juncea resulted in the best results, in relation to hairy vetch and fallow, not corroborating the present study, since the crotalaria and vetch crops are in the same group (S2).