Agricultural intensification may often result in a deterioration of soil quality, which in turn affects soil productivity. Management practices can have a significant impact on the composition of soil biota (Luque et al., 2005; Gomez et al., 2007). With regards to no-tillage (NT) systems, Beare et al. (1993) established that in soils under no-tillage, macroaggregates constitute an important mechanism for the protection of soil organic matter (SOM), which is otherwise mineralized by the frequent disruption of aggregates under conventional tillage (CT). The amount of crop residue on the soil surface is increased (providing substrates for the growth of fungal, bacterial and yeast populations) and influenced by the soil environment (changes in the moisture and temperature). CT causes a drastic modification of the soil environment. Excessive tilling is manifested by a marked deterioration of the soil structure and a progressive acidification. Wonisch et al. (1995) observed that, in soils under CT, the composition of the SOM and the diversity and size of the soil microbial communities were reduced. Gupta et al. (1994) established that stubble burning and excessive tillage caused important losses of SOM in many parts of the world. The same authors also reported that crop residue retention and reduced tillage practices can reverse these trends and increase nutrient inputs to the soil.

Nitrogen (N) is one of the nutrients recycled through crop residues. Most of the N in crop residues is in organic form and is not directly available for plant growth. During decomposition of crop residues, this organically bound N is made available for crop or microbial growth through N mineralization.

The amount of N that is mineralized or immobilized during the decomposition of crop residues influences its availability for crops and ultimately impacts on N management practices. The dynamics of soil mineral N, associated with residue decomposition, is affected by the type and position of the residue and by the concentration of N (Sanchez et al., 1998). The variation in the chemical composition of the residues throughout the time also affects microbial activity and N dynamics (Green et al., 1995; Sanchez et al., 1996). The addition of N through fertilization increases the decomposition rate of stubble, which in turn affects N availability in the soil (Blackmer & Green, 1995).

By comparing different cropping systems (native pastures and research plots with different crop intensities), Acosta Martinez et al. (2007) found that natural pastures produce between two- and five-fold greater microbial biomass carbon and between three- and seven-fold greater microbial biomass nitrogen than a disturbed cropping condition. These authors added that fallow periods and CT negatively affect soil microbial biomass.

The soil decomposer community consists of a wide range of bacteria, fungi, protists and invertebrates. Saprophytism is the dominant life style among these soil microorganisms and saprophytic fungi are essential to the maintenance of the arable soil ecosystem in terms of nutrient turnover. Fungi are the most important agents in the production of dissolved organic matter, probably because of their incomplete degradation (Kalbitz et al., 2000). While studying changes caused by stubble placement in three tillage systems (chisel tillage, no tillage and conventional tillage), Toresani et al. (1998) concluded that no tillage had the highest number of fungal cellulolytic organisms at 0-5 cm and 5-10 cm of depth. Gomez et al. (2007) found that the greatest fungal diversity was associated with no-till soils and mentioned that the genera Cladosporium and Fusarium were related to no-till and undisturbed sites. These genera survived in surface residues and slightly degraded organic matter. The same authors added that Penicillium and Aspergillus were the most frequent and abundant genera found in all sites, except in no-till, and that Trichoderma spp. were abundant in conventionally cropped and chiseled sites, suggesting that this genus can survive in tilled soils. Into fungal group, arbuscular mycorrhizal fungus (AMF) is crucial for plant growth and health. According to Wetzel et al. (2014) tillage negatively affect AMF diversity even under conditions of intensive agricultural management. A large number of studies have demonstrated a clear negative correlation between land use intensity and the diversity of AMF communities. Schalamuk & Cabello (2010), established that both NT and CT may influence the AMF propagules bank composition. Numerous studies have shown that mycorrhizal colonization is negatively affected by tillage (Schalamuck et al., 2003, 2004). A different result was obtained with the effect of the propagules and the fungicide application. Schalamuk et al. (2014) for Argentinian conditions reported that the Mycosphaerella graminicola fungicide treatment actually favored the formation of arbuscules and AMF spores, with a selective increase in the density of the glomoid morphotype.

We hypothesized that agricultural practices such as cropping, tillage and fertilization affect the size and composition of the soil fungal communities, and that the effects of these practices vary along the time. The objective of this research was to investigate the effect of disturbed (cropped) and undisturbed (uncropped) soils under different tillage practices and N fertilization on the soil fungal populations associated with wheat stubble degradation in a Luvic Phaeozem of Argentina.

The use of Nash Snyder as specific culture medium allowed the growth of Fusarium spp. and other fungal species. In contrast, Oxgall agar medium allowed to quantify more species, with greater diversity values than the former, for the three sampling times.

The ANOVA for CFU/g of soil (data not shown) revealed that the sampling times were highly significant for the two culture media (p<0.01) and for the “sampling times × tillage” interaction at 0.05 and 0.01 for Nash Snyder and Oxgall agar, respectively. The results of the factorial analysis for each sampling time using CFU/g of soil values showed that in both culture media, the tillage system was highly significant at post-harvest. In addition, before sowing and at tillering, significant differences at 5% were found when CFU were measured in Nash Snyder (Table 1). Figure 2 shows that the CFU/g of soil counted on Oxgall agar showed a significant “tillage × fertilization” interaction only at post-harvest. Tukey’s test demonstrated that the samples found under RT and N160 had values of CFU significantly higher than CT-N160 (**p< 0.01) but not significantly different from RT-N0. In contrast, soil samples from CT with N160 had the lowest number of propagules.

The analysis of each source of variation and the three sampling dates are shown in Figure 3 (a-f). The highest values of CFU/g of soil were found for the samples collected at post-harvest from soils under RT and differed significantly from those at CT throughout the experiment (Fig. 3a). The fungal colonies growing on Nash Snyder also produced higher values of CFU/g of soil when the samples belonged to soils under CT, both before sowing and at tillering (Fig. 3b). At post-harvest, the values of CFU/g of soil obtained from soil samples taken from NOCA and N0 differed significantly from those of the other treatments in both culture media (Fig. 3c, d, e, f).

Our results demonstrated that, for both culture media, the CFU values of samples coming from RT decreased along the sampling times and that those from CT increased slightly (Fig. 3a, b).

Table 2 shows the significance of H values on Oxgall agar medium and post-harvest time. The factorial analysis shows significant differences between tillage systems (**p<0.01), fertilization levels (*p<0.05) and cropped condition (*p<0.05), and for the “tillage × fertilization” and “tillage × cropping condition” double interactions (*p<0.05). The value of H was significantly reduced (Tukey’s test *p<0.05) (Fig. 4a, b) when the CFU values were obtained at post-harvest from soil sampled on CT and N160 or CT and CA.

Table 3 shows the fungal relative abundance of each genus and species on soils adjacent to the stubble litterbags for each tillage system and sampling time. In soil samples taken at post-harvest, when the wheat residues were fresh, most fungal taxa named “specialists” were preferentially associated with soil coming from surface or buried residues. In RT, the “specialists” fungi were Fusarium equiseti, Nigrospora sp., Phoma sp., Acremonium sp., and Sepedomium sp., whereas in CT they were yeasts, F. graminearum, Ulocladium sp., Alternaria triticina, Arthrinium sp. and Chaetomium cochliodes. Other soil fungal species identified from both tillage systems were named as “generalists”. These included Trichoderma spp., F. oxysporum, Aspergillus spp., Penicillium spp., Alternaria alternata, Mucor racemosus f. racemosus and Cladosporium cladosporioides.

Soil tillage induces many physical, chemical and biological changes. Important modifications in soil processes are due to the location of crop residues in the soil profile, with crop residues remaining on the surface in no tillage and with incorporation of residues by plowing in conventional tillage. Surface stubble residues reduce soil temperature oscillations, keeping the soil cool, maintaining soil moisture during the hot and dry seasons, and promoting microbial activity and crop development.

Although our results are descriptive of only some portion of the microbial community (only the fungal community), they clearly show differences in fungal genera and species and their relative abundance throughout the sampling period.

The soil viable fungi were counted using the dilution plate count technique, as previously done by Luque et al. (2005), Mesci et al. (2006) and Gomez et al. (2007) for Argentina and by Harris et al. (1994) for the US. We recognize that this method is imperfect because it detected only part of the total community and was subject to quantitative error. Some species would be inhibited by other fungi that produce large numbers of spores and more quickly, without being detected by the dilution method. The Oxgall agar, which has a fungistatic substance, added to the basal PDA medium used in this study, reduced the growth of the fast growing species.

Based on our erlier experience, to apply in this study the washing method (Parkinson & Williams, 1961) would very difficult to obtaining results fast. The reason is the number and frequency of the soil samples to manage in the experiment. However, the dilution plate technique appeared to be the method used more frequently to study the distribution of the cellulolytic population of the soil mycoflora (Biederbeck & Campbell, 1971; Pfender & Wootke, 1988; Beare et al., 1993; Harris et al., 1994; Nesci et al., 2006; Gómez et al., 2007). The soil dilution technique allowed to obtain a continuous number of CFU of different genera in a short period of time. Considering that the detectable portion of the mycoflora differs over time and environments, this method resulted useful in this research to indicate changes in the community composition (Pfender & Wootke, 1988).

Changes in the number of propagules, as was observed in this work, are expectable when taking into account the period of time between sampling dates. In this sense, Broder & Wagner (1988) established that although many of the temporal changes in fungal composition may be attributed to succession during residue decay, some are probably due to seasonal changes in temperature and humidity, which influence the growth of some fungal species. Under the experimental conditions of this research, the changes in temperature (soil temperature) and humidity (soil relative humidity) were less pronounced in CT than in RT and the fungal communities maintained their biological equilibrium, increasing their CFU/g of soil both before sowing and at tillering. Our results add to the conclusions of Luque et al. (2005), who affirmed that the fungal counts were higher in the top layer of the soil profile for the abundance of stubble in the surface with important oxygen availability. In this sense, Kandeler & Bohm (1996) verified that reduced and minimum tillage provides enough crop residues and organic C as substrate for multiplication of soil microorganisms.

The association between the number of propagules and the high doses of fertilization, are in agreement with the results by Broder et al. (1984), Linn & Doran (1984) and Reicosky & Lindstrom (1993). Our results indicate that in CT, the low number of CFU/g of soil could be related to more rapid N mineralization and nitrification with increase N leaching from the surface of fertilized soil. The responsible of this phenomenon is the characteristic and abundant fungal cellulolytic population appeared in CT, that lives in the soil matrix in buried conditions. Under this condition, we suggest that N increased as consequence of degradation of soil residues. It was added to N incorporated by fertilization. Thus the activity of the cellulolityc mycoflora accelerated drastically, decreasing the nutritional basis of the fungal population and as consequence, decreased the CFU/g of our soils.

Regarding the CFU/g of soil under NOCA, the largest total number of fungal colonies could be due to the higher soluble organic carbon (SOC) and total N found under native soil conditions. The result is consistent with other findings. Acosta Martinez et al. (2007) established that native pastures showed higher SOC, microbial biomass and enzyme activities than CA, likely due to differences in their root systems and root exudates. Sall et al. (2006) analyzed total microbial biomass and compared plots from a 21-year fallow and other cultivated for 4 years after leaving fallow for 17 years. The cultivated soils had lower C, N and P concentrations than the fallow soil. Also, the number of fungi and bacteria was, on average, twice and ten times higher in the fallow soil than in the cultivated soil. The microbial community in the fallow soil had all the enzymes necessary for the immediate degradation of the soluble compounds in the litter. Similar results were published by Garcia Orenes et al. (2010) and Nautiyal et al. (2010) for a semi-arid ecosystem.

The higher value of H from the soil under RT compared with CT confirmed the adverse effect of tillage on diversity. These results are consistent with those reported by Hassinsk et al. (1991) and Wander et al. (1995), who used different methods to evaluate microbial diversity. The results of this work agree with previous reports from Nesci et al. (2006) and Silvestro et al. (2013), who observed that Fusarium spp. is one of the most abundant fungi in agricultural soils and grasslands. Also, the surface residue accumulation due to CT can promote the survival of pathogens like Fusarium. In agreement with that found by Silvestro et al. (2013), we observed that F. oxysporum was the most abundant species in all cropping conditions, tillage systems and sampling dates, with a slight decrease near tillering. The highest value of this species was observed at post-harvest and RT (18%), whereas the lowest was observed at tillering and RT (12%) or CT (0.4%). The high density of F. oxysporum in agricultural soils is a matter of study because this fungus develops pathogenic and non-pathogenic isolates. F. equiseti was simultaneously isolated with F. oxysporum and F. graminearum but in a smaller abundance. The highest value was associated with before sowing and CT (10%) and the lowest with post-harvest and before sowing and RT (1.2%). Some of the fungal genera identified, such as Aspergillus spp. and Penicillium spp., were not affected by the tillage system, similar to that documented by Nesci et al. (2006). In contrast, Gomez et al. (2008) cited that Penicillium was predominant in undisturbed and native pasture and that Aspergillus was isolated from all different conditions and management practices. Gomez et al. (2007) reported that the genera named generalists in this work, except Cladosporium and Alternaria, are involved in organic matter decomposition. Fusarium and Cladosporium were significantly higher in non-disturbed and no-till sites, while Alternaria was found only in the cropped system. This genus showed the lowest frequency and abundance because it is not a soil-borne fungus. Most of the species isolated are cosmopolitan and can be found in cultivated soils and in diverse organic material, mainly on plant residues in different states of decomposition. Also, Trichoderma spp. have shown their capacity to degrade cellulosic materials (Domsch et al., 2007). Among the fungal genera evaluated, Fusarium, Cladosporium and Alternaria included phytopathogenic species, as mentioned by Gomez et al. (2007), while Penicillium and Trichoderma were considered antagonistic organisms. According to the present results and in agreement with those found by Beare et al. (1993), Gomez et al. (2007) and Vargas Gil et al. (2008), the population of Trichoderma spp. of the soil studied were most abundant and prevalent in no-tillage or reduced tillage systems. Moreover, Luque et al. (2005) reported that Trichoderma was associated as antagonist against Fusarium spp. been relatively more frequently isolated in the cropping systems as in conventionally and chiseled sites. Maintaining a diverse and balanced population of biologically active microbial species, throughout management practices, creates a healthy soil and contributes to improved crop production.