In this paper, soil carbon, nitrogen and phosphorus concentrations and stocks
were investigated in agricultural and natural areas in 17 plot-level paired
sites and in a regional survey encompassing more than 100 pasture
soils In the paired sites, elemental soil concentrations and stocks were
determined in native vegetation (forests and savannas), pastures and
crop–livestock systems (CPSs). Nutrient stocks were calculated for the soil
depth intervals 0–10, 0–30, and 0–60 cm for the paired sites and
0–10, and 0–30 cm for the pasture regional survey by sum stocks
obtained in each sampling intervals (0–5, 5–10, 10–20, 20–30, 30–40, 40–60 cm). Overall, there were significant differences in soil element
concentrations and ratios between different land uses, especially in the
surface soil layers. Carbon and nitrogen contents were lower, while
phosphorus contents were higher in the pasture and CPS soils than in native
vegetation soils. Additionally, soil stoichiometry has changed with changes
in land use. The soil C : N ratio was lower in the native vegetation than in
the pasture and CPS soils, and the carbon and nitrogen to available
phosphorus ratio (P
The demand for food will continue to grow in order to feed a population that will reach near 9 billion people worldwide in 2050 (Tilman et al., 2011). Brazil is one of the pivotal countries that will have a key role in the global food production system (Martinelli et al., 2010). There is already a consensus that an increase in food production cannot be achieved by replacing native vegetation with agricultural fields (Tilman et al., 2011). One of the alternatives that has been proposed is agricultural intensification, which means not only an increase in productivity but also an attempt to increase sustainability (Godfray et al., 2010). Sustainable agriculture (SA) has been proposed as one way to achieve both goals. SA tries to mimic natural ecosystems by adding layers of complexity in an attempt to depart from simplistic monoculture fields (Keating et al., 2010).
Crop–livestock systems (CPSs) are a suitable example of this attempt to add a layer of complexity to agricultural fields. Integrated crop–livestock or crop–livestock–forest, and agroforestry systems are not a new idea. However, these systems have only been consolidated in recent decades (Machado et al., 2011). The system consists of diversifying and integrating crop, livestock and forestry systems, within the same area, in intercropping, in succession or rotation. The system can provide environmental benefits such as soil conservation, building up soil carbon, reducing environmental externalities and ultimately increasing productivity. CPSs include but are not restricted to the following: no-till, the use of cover crops, elimination of agricultural fires (slash and burn), and restoration of vast areas of degraded pastures (Machado et al., 2011; Bustamante et al., 2012; Lapola et al., 2014). Additionally, the Brazilian law (Law no. 12187 of 29 December 2009) encourages the adoption of good agricultural practices to promote low carbon emission (Low Carbon Emission Program – ABC Program) and stipulates that mitigation should be conducted by adopting (i) recovery of degraded pastures, (ii) a no-tillage system, (iii) integrated livestock–crop–forest systems, and (iv) re-forestation in order to reduce approximately 35 to 40 % of Brazil's projected greenhouse gas emissions by 2020 (Assad et al., 2013).
The CPSs have been evaluated in several ways, especially regarding soil
carbon balance with cultivation (Sá et al., 2001; Ogle et al., 2005;
Zinn et al., 2005; Bayer et al., 2006; Baker et al., 2007). On the other
hand, there are few regional studies considering how nitrogen and phosphorus
soil contents will be affected in these integrated agricultural systems.
Plot-level studies have reported a decrease in soil nitrogen stocks with
cultivation in several N-fertilized areas of Brazil and under different
cropping systems (Lima et al., 2011; Fracetto et al., 2012; Barros et al.,
2013; Sacramento et al., 2013; Cardoso et al., 2010; Silva et al., 2011;
Guareschi et al., 2012; Sisti et al., 2004; Santana et al., 2013; Sá et
al., 2013). The same trend has been observed in Chernozem soils in Russia and
in prairie soils of Wisconsin in the USA (Mikhailova et al., 2000; Kucharik
et al., 2001). In unfertilized pasture soils of Brazil, nitrogen
availability decreased as the age of pastures increased. In theses soils,
there was an inversion in relation to forest soils, and an ammonium
dominance over nitrate was observed, followed by lower mineralization and
nitrification rates that in turn were followed by lower emissions of
N
Phosphorus is particularly important in the tropics due to phosphorus adsorption on oxides and clay minerals rendering it unavailable to plants (Uehara and Gillman, 1981; Sanchez et al., 1982; Oberson et al., 2001; Numata et al., 2007; Gama-Rodriguez et al., 2014). This P adsorption, as well as the fact that phosphorus does not have a gaseous phase like nitrogen, renders phosphorus less mobile in the soil–plant–atmosphere system than nitrogen (Walker and Syers, 1976). One consequence of this lower phosphorus mobility throughout the soil profile is that when P fertilizers are applied, they tend to increase soil phosphorus concentration on the soil surface, but they also make phosphorus available by loss through the soil erosion process and surface runoff (Messiga et al., 2013). The use of agricultural practices like no-till may further increases phosphorus concentration in the surface soil due to the non-movement of the soil layer (Pavinatto et al., 2009; Messiga et al., 2010, 2013). Soil phosphorus is also affected by physical characteristics of the soil, such as how the size of soil aggregates influences the extent of soil phosphorus availability to plants (Fonte et al., 2014). Therefore, agricultural practices have the potential to alter soil phosphorus concentration and consequently soil phosphorus stocks (Tiessen et al., 1982; Tiessen and Stewart, 1983; Ball-Coelho et al., 1993; Aguiar et al., 2013).
Besides concentrations and stocks, agricultural management is also capable of altering the ratios between carbon, nitrogen and phosphorus (C : N : P; Tiessen et al., 1982; Tiessen and Stewart, 1983; Ding et al., 2013; Jiao et al., 2013; Schrumpf et al., 2014; Tischer et al., 2014). For instance, soil microorganisms adjusting their stoichiometry with that of the substrate may release or immobilize nitrogen depending on the substrate C : N ratio (Walker and Adams, 1958; Mooshammer et al., 2014a). In turn, litter decomposition also depends on the stoichiometry of the litter, especially on the C : N ratios (Hättenschwiler et al., 2011). These adjustments guided by C : N : P ratios may ultimately interfere in crop production, which in turn will affect soil carbon sequestration and, consequently, agro-ecosystem responses to climate change (Hessen et al., 2004; Cleveland and Liptzin, 2007; Allison et al., 2010).
Agricultural land in Brazil has increased dramatically over recent decades and part of this increase contributed to increase deforestation rates in all major Brazilian biomes (Lapola et al., 2014). Particularly important in Brazilian agriculture is the area covered with pasture that includes approximately 200 million hectares encompassing degraded areas with well-managed pasture (Martinelli et al., 2010). Arable land comprises almost 70 million hectares, with approximately 30 million hectares under no-till cultivation (Boddey et al., 2010), with CPSs being especially important in the southern region of the country.
Most studies in Brazil on the effects of agricultural practices on soil properties deal with soil carbon stocks due to its importance for a low-carbon agriculture (Sá et al., 2001; Bayer et al., 2006; Marchão et al., 2009; Maia et al., 2009; Braz et al., 2012; Assad et al., 2013; Mello et al., 2014). On the other hand, there are fewer studies on agricultural practices affecting soil nitrogen concentration, and especially stocks, and even fewer studies on changes in soil phosphorus stocks. Based on this, this paper aims to investigate effects of agricultural practices on carbon, nitrogen and phosphorus soil concentration and stocks, plus the soil stoichiometry (C : N : P ratio), in several Brazilian regions using the same study sites and methodology used by Assad et al. (2013), who evaluated changes in soil carbon stocks due to different land uses. Two sampling approaches were used in Assad et al. (2013): the first, at the plot level, addressed 17 paired sites, comparing soil stocks among native vegetation, pasture and CPSs, and the second was a regional survey of pasture soils in more than 100 sites.
A full description of the study area can be found in Assad et al. (2013).
Briefly, we conducted two types of surveys: one at the regional level,
exclusively in pasture soils, and another in which 17 plot-level
paired sites were sampled encompassing soils of pastures, CPSs and native vegetation. The regional pasture survey was
conducted in November and December of 2010, and 115 pastures located between
6.58 and 31.53
Sampling sites located throughout Brazil. White circles indicate pasture sites of the regional survey; black circles indicate paired study sites, and various shaded areas indicate Brazilian biomes.
Paired sites were selected by the EMBRAPA (Empresa Brasileira de Pesquisa
Agropecuária) regional offices and sampled between November and December
2011. At these sites, there was an attempt to sample areas of native
vegetation, pasture and sites that encompass crop rotation integrated with
livestock (CPSs). A detailed description of crop rotation and sites that
combine crops and livestock management is shown in Table 1. Native vegetation is composed of wood vegetation in the Atlantic Forest or Cerrado biome characteristics. In sites located in the southern region of the country
(Arroio dos Ratos, Tuparecetã, Bagé, and Capão do Leão), the
original vegetation is grassy temperate savanna locally referred to as
Campos, which belongs to the Pampa biome (Table 1). For the sake of simplicity,
forests and Campos soils were grouped under the category named “native
vegetation”. Pasture was composed mostly of C
Characterization of sampled sites: native vegetation (NV), pastures
(
Continued.
Soil sampling is described in detail in Assad et al. (2013). Briefly, in
each site, a trench of 60 by 60 cm, yielding an area of approximately 360 cm
Air-dried soil samples were separated from plant material and then
homogenized. The samples were then run through sieves for chemical and
physical analysis (2.0 mm sieve diameter) and analysis of soil carbon (0.15 mm sieve diameter). The concentration of soil nitrogen and carbon, which may
also include fine charcoal, was determined by using the elemental analyzer
at the Laboratory of Isotopic Ecology Center for Nuclear Energy in
Agriculture, University of São Paulo (CENA-USP) in Piracicaba, Brazil.
Phosphorus concentration was determined by extracting soil phosphorus using
the Mehlich-3 method of extraction (Mehlich, 1984), and phosphorus
concentration was quantified by the colorimetric blue method. Accordingly,
the C : P and N : P ratios shown here did not use organic phosphorus (P
Carbon stocks were reported in Assad et al. (2013). In this paper, besides
carbon concentrations, nitrogen stocks expressed in Mg ha
The cumulative soil nitrogen and phosphorus stocks for fixed depths were
calculated by the following equation:
In order to test for differences in element concentrations and their respective ratios, we grouped element contents by land use (forest, pasture, CPS) and soil depth (0–5, 5–10, 10–20, 20–30, 30–40, 40–60 cm). Carbon, nitrogen and phosphorus concentration, and soil nitrogen and phosphorus stocks must be transformed using Box–Cox techniques because they did not follow a normal distribution. Accordingly, statistical tests were performed using transformed values, but non-transformed values were used to report average values. The element ratio was expressed as molar ratios, and ratios followed a normal distribution and were not transformed.
For the paired sites, differences between land uses (native vegetation, CPS
and pasture) were tested with ANCOVA, with the dependent variables being
transformed nutrient concentrations at the soil depth intervals described
above, and stocks at the soil layers of 0–10, 0–30, and 0–60 cm;
the independent variables were land-use type. As mean annual temperature
(MAT), mean annual precipitation (MAP), and soil texture may influence soil
nutrient concentration, ratios, and stocks, these variables were also
included in the model as co-variables. The post hoc Tukey honest significant test for unequal
variance was used to test for differences among nutrient stocks of different
land uses. In order to determine whether changes in soil nutrient stocks
between current land use and native vegetation were statistically
significant, we used a one-sample
Soil depth variability of
Carbon, nitrogen, and phosphorus concentrations decreased with soil depth
(Fig. 2). The average carbon concentration was higher in the topsoil (0–5
and 5–10 cm) of native vegetation soils compared with pasture and CPS soils
(
The C : N ratios of pasture and CPS soils were higher than the native
vegetation soils in all soil depths; however, this difference was not
statistically significant for any particular depth (Fig. 3a). There was a
difference in the C : P
Soil depth variability of
The average nitrogen stock of the native vegetation soils in the topsoil was
2.27 Mg ha
Mean, standard deviation (SD), and minimum and maximum of soil
nitrogen stocks (N
On the other hand, a net gain of phosphorus was observed between native
vegetation and current land uses in the soil. The phosphorus soil stock in
the topsoil of native vegetation areas was equal to 11.27 kg ha
Mean, standard deviation (SD), and minimum and maximum of soil
phosphorus stocks (P
Statistics of soil nitrogen (N
We compared element concentrations and ratios of the regional survey pasture
soils with the native vegetation soil site of the plot-level paired sites
(Figs. 2 and 3). Carbon, nitrogen and phosphorus concentrations decreased
with soil depth, and were significantly lower (
At the 0–10 cm soil layer the average total soil nitrogen stock was equal
to 1.66
Due to time and financial constraints, we were unable to sample soil from native vegetation near each pasture site in the regional survey. This poses a challenge because it is important to compare changes in the soil nitrogen and phosphorus stocks with the native vegetation as done in the paired study sites. In order to overcome the lack of original nutrient soil stocks, we used estimates of native vegetation obtained in the paired sites. Another difficulty was the lack of reliable information on the land-use history; we cannot guarantee that differences among land uses already existed or were due to the replacement of the native vegetation (Braz et al., 2012; Assad et al., 2013). In addition, we only have a point-in-time measurement; we did not follow temporal changes in nitrogen and phosphorus soil stocks. Therefore, it is not possible to know whether the soil organic matter achieved a new steady-state equilibrium; as a consequence our results should be interpreted with caution (Sanderman and Baldock, 2010).
Scatter plot of soil carbon stock losses (data from Assad et al.,
2013), and soil nitrogen stock losses found in this study between CPS and
native vegetation in the paired study sites
Overall, the C : N ratio was lower in the native vegetation soils compared with pasture and CPS soils (Fig. 3a). These differences are probably explained by a nitrogen loss and not a carbon gain, since soil carbon stocks in pasture and CPS soils were lower than in native vegetation soils (Assad et al., 2013). Lower soil C : N ratios as observed in the native vegetation could influence nitrogen dynamics, favoring faster organic matter decomposition and nitrogen mineralization by microorganisms in these soils (Mooshammer et al., 2014b). However, it is difficult to conclude whether a small difference between native vegetation soils and the others would be enough to alter the balance between mineralization and immobilization, especially because Mooshammer et al. (2014a) showed that microbial nitrogen use efficiency has a large variability in mineral soils.
Another important trend was the lower depth variability of C : N ratios
compared with the depth variability of carbon and nitrogen concentrations
(Fig. 2a and b). This trend is consistent with the initial hypothesis of
Tian et al. (2010), who hypothesized that the C : N ratio would not vary widely
with depth because of the coupling of carbon and nitrogen in the soil.
According to Tischer et al. (2014), such constancy is a consequence of
similar inputs of organic matter by primary producers to the soils and also
due to the fact that N transformations (immobilization or mineralization)
are coupled to C transformations, especially when soil organic carbon
molecules are converted into CO
Scatter plot of soil carbon stock losses (data from Assad et al.,
2013), and soil nitrogen stock losses found in our study between pasture and
native vegetation in the paired study sites
Among different land uses, the elements: P
In most of the plot-level paired sites and in most of the regional soil survey, we found a loss of nitrogen compared to the native vegetation. It seems that this is a common pattern observed for different crops and different types of land management in several regions of Brazil, like in the northeast (Lima et al., 2011; Fracetto et al., 2012; Barros et al., 2013; Sacramento et al., 2013), in central Brazil (Cardoso et al., 2010; Silva et al., 2011; Guareschi et al., 2012) and in the south (Sisti et al., 2004; Sá et al., 2013; Santana et al., 2013). Sá et al. (2013) found lower soil nitrogen stocks in several farms located in southern Brazil (Paraná State) that have adopted no-till and crop rotation systems for at least 10 years compared with the native vegetation of the region. On the other hand, the adoption of no-till systems tends to increase soil nitrogen stocks compared to conventional tillage (Sisti et al., 2004; Sá et al., 2013). In this respect, it is interesting to note that the only three sites (SL, PG, AP) where the soil nitrogen stocks were higher in the agriculture field than in the native vegetation were CPS sites, where no-till was practiced and there was a system of crop rotation, with soybean in the summer and oat or wheat in the winter (Table 1).
Absolute difference of soil nitrogen stocks between different
depth intervals:
Nitrogen dynamics are regulated by a balance between inputs, losses and transformations between different forms of nitrogen (Drinkwater et al., 2000). Regarding nitrogen inputs, the main natural nitrogen input is via biological nitrogen fixation (BNF), and the main anthropogenic addition is via N-mineral fertilizer inputs (Vitousek et al., 2002). In crops like soybean, BNF is also important as a source of new nitrogen to the system, especially in Brazil, where soybean may fix higher amounts of nitrogen (Alves et al., 2003). Several of the CPSs evaluated in this study involve the use of soybean under crop rotation systems (Table 1); however, decreases in soil nitrogen stocks of these CPSs were also observed in these systems (Fig. 6a and b). The same was observed by Boddey et al. (2010) comparing soil carbon and nitrogen stocks of no-till and conventional tillage systems involving a crop rotation with soybean in farms located in Rio Grande do Sul State (southern Brazil). According to these authors, the nitrogen export by grain harvesting is high enough to prevent a buildup of this nutrient in the soil (Boddey et al., 2010).
Absolute difference of soil phosphorus stocks between different
depth intervals:
On the other hand, most pastures in Brazil are not fertilized, so over time a decrease in nitrogen inputs coupled with an increase in nitrogen outputs is generally observed, leading to lower mineralization and nitrification rates (Verchot et al., 1999; Melillo et al., 2002 Garcia-Montiel et al., 2000; Wick al., 2005; Neill et al., 2005; Carmo et al., 2012). According to Boddey et al. (2004), not even the return of nitrogen to soil pasture via urine and dung is sufficient to compensate for other nitrogen losses. As a consequence, the continuous use of unfertilized pastures leads to overall N impoverishment in the system, leading to lower soil nitrogen stocks, as observed in this study.
We found a positive and significant (
On the other hand, we observed a general increase in soil phosphorus stocks of pasture and CPS paired sites compared with soil stocks of the native vegetation (Fig. 7a and b). The higher soil phosphorus stocks in the CPS could be explained by the addition of phosphorus fertilizer to the fields (Aguiar et al. 2013; Messiga et al., 2013; Costa et al., 2014). Generally, an increase in soil phosphorus is observed after use of P fertilizers in the topsoil due to the low mobility of phosphorus, especially in no-till systems (Costa et al., 2007; Pavinatto et al., 2009; Messiga et al., 2010). In several of the CPS sites, there are crop rotations of maize, rice and soybean, and all these crops are fertilized with phosphorus, especially soybean, because phosphorus is an important nutrient in the biological nitrogen fixation process (Divito and Sadras, 2014). The variation in phosphorus concentration with soil depth provides indirect support for this hypothesis. In the majority of the CPS sites and even pasture soils of the paired sites, there is a gradient in phosphorus concentration, with much higher concentrations near the soil surface (Fig. 2c).
The soil phosphorus stocks of pastures located in the paired sites were
higher than soil phosphorus stocks of the regional pasture survey. For
instance, in the 0–10 cm soil layer, the average P
In an earlier paper, Assad et al. (2013) showed a decrease in soil carbon stock in relation to the original vegetation either for pasture and CPS soils. In this paper we found that nitrogen stocks also decrease considerably with land-use changes, even in well-managed CPSs, and especially in pastures of the regional survey that reflect better the reality of pasture management in Brazil. These findings have important policy implications because Brazil recently implemented a program (Low Carbon Agriculture) devoted to increasing carbon and nitrogen concentration in soils through a series of techniques, especially no-till, CPSs, and improvement of degraded pastures. Therefore, the findings of this paper set a baseline of soil nutrients stocks and stoichiometry for future comparisons.
We would like to thank the British Embassy for financial support. Jim Hesson
of