Soil quality and its relationship with weeds in urban homegardens of Alta Floresta, southern Amazonia

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Soil quality and its relationship with weeds in urban

homegardens of Alta Floresta, Southern Amazonia

Wagner Gervazio.Oscar Mitsuo Yamashita.Ricardo Adriano Felito. Pedro V. Eisenlohr

Received: 9 January 2017 / Accepted: 16 April 2018 / Published online: 20 April 2018 Ó Springer Science+Business Media B.V., part of Springer Nature 2018

Abstract Understanding the possible causes of dif-ferences in weed composition of homegardens is important to manage such environments and, thus, to control the propagation of weeds. One of the factors that could influence the differences in weed compo-sition of homegardens is soil quality. Soil is a complex and dynamic component subject to change; adequate management is therefore indispensable. The objective of this study was to evaluate the relationship between soil quality and weed species in 30 urban homegardens in Southern Amazonia. We collected samples of the

soil seed bank of weeds, calculated quantitative indices of these weed species, and performed an analysis of soil variables (pH, phosphorus, potassium, calcium, magnesium, aluminum, hydrogen, organic matter, sum basis, cation exchange capacity, base saturation, and soil texture). We conducted a principal components analysis to condense the information contained in the soil variables in a small group of new dimensional compositions, a detrended correspon-dence analysis to investigate the floristic gradients, and a redundancy analysis to model the influence of soil variables on variation in weed spectrum. The soils under the studied urban homegardens showed a chemical quality mostly above the desired minimum of macronutrients, pH, and base saturation. Based on the correlation between the presence of weeds and the physico-chemical quality of the soil, we must recog-nize that we are still far from obtaining a satisfactory model that can explain the composition of weeds and their relation with soil fertility in urban homegardens.

Keywords Urban agroforestry systems Multivariate analysis Seed bank Soil variables

Introduction

Urban homegardens can be considered an extension of the household and aim to sustainably produce foods Electronic supplementary material The online version of

this article (https://doi.org/10.1007/s10457-018-0230-x) con-tains supplementary material, which is available to authorized users.

W. Gervazio

Programa de Po´s-Graduac¸a˜o em Biodiversidade e Agroecossistemas Amazoˆnicos, UNEMAT, Alta Floresta, MT 78580-000, Brazil

Present Address: W. Gervazio

Programa de Po´s-Graduaçaõ em Engenharia Agrıćola, Facul-dade de Engenharia Agrıćola, UNICAMP, Avenida Caˆndido Rondon, 501, Baraõ Geraldo, Campinas, SP 13083-970, Brazil O. M. Yamashita R. A. Felito P. V. Eisenlohr (&) Programa de Po´s-Graduaçaõ em Biodiversidade e Agroecossistemas Amazoˆnicos, UNEMAT, Campus II -Avenida Perimetral Roge´rio Silva, 4930 - Jardim Flamboyant, Alta Floresta, MT 78580-000, Brazil e-mail: pedro.eisenlohr@unemat.br

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that significantly complement the family diet (Nair

1986). Generally, a wide variety of plant species and small animals interact in such an environment (Kabashima et al.2009). Homegardens also contribute to food security and to the conservation of local agricultural biodiversity (Kumar 2016). Understand-ing the possible causes of differences in weed composition of homegardens is important to manage such environments and, thus, to control the propaga-tion of weeds. For example, there are situapropaga-tions in which the coexistence among weeds is favored (Anzalone et al.2012). However, despite the relevance of investigating the drivers of weed composition variation in homegardens, this issue has been poorly addressed worldwide.

One of the factors that could influence the differ-ences in weed composition of homegardens is the soil quality (Coomes and Ban 2004), which can be analyzed to improve our understanding of the dynam-ics of urban homegardens and their productivity. Soil quality can be understood as the ability of a given type of soil to work within the limits of managed or natural ecosystems, sustaining plant and animal productivity, maintaining air and water quality, and promoting human health (Doran and Parkin1994). The interest in assessing soil quality is based on the awareness that soil is one of the most important regulators of terrestrial cycles and a crucial factor for global productivity (Vezzani 2001). Soil is a complex and dynamic component subject to change; proper man-agement is therefore indispensable (Gliessman2005). In this respect, aboveground biomass can have direct and indirect effects according to soil quality and fertility, through tree size inequality and, or, species diversity (Ali and Mattsson2017). Both soil nutrient availability and quality play an important role in determining aboveground biomass, and have strong direct effects on soil (Zhang and Chen 2015), with species productivity and diversity also varying accord-ing to the environmental conditions (Poorter2015).

Previous studies highlighted that weeds are indica-tive of soil quality and fertility, reflecting management conditions (Primavesi 1992; Embrapa 2006) by homegardeners (Diekmann 2003). In addition, the use of biological indicators becomes possible because the systems have direct or indirect relationships with the environment (Zonneveld1983). In several studies, the bioindication through the presence or absence of plants in a given location has been studied by

evaluating fertility and moisture characteristics of soil (Host and Pregitzen1991; Wilson et al.2001; Ewald et al.2013; Ponette et al.2014). In fact, the evaluation of soil quality through the relation with the presence of weeds and environmental factors can be useful for the establishment of sustainable agriculture (Reichert et al.2003). In this context, knowledge of soil quality and its evaluation through physical, chemical, and biological indicators, as well as investigations on the relationship between soil parameters and weeds com-position, are of great importance (Reichert et al.2003). In this study, we evaluated chemical indicators, soil texture, and biological attributes to determine whether the presence of weeds in urban homegardens may be an indicator of soil quality (Primavesi1992), consid-ering also the perception of the urban homegardeners. Specifically, we evaluated the soil quality parameters and its relationship with the presence of native weed species in urban homegardens, addressing whether a relationship between soil quality and the presence of weeds in urban homegardens could be consistent in a municipality of Southern Amazonia. We hypothesized that homegardens would play an important role in maintaining soil fertility and, consequently, better conditions for crop development and increased food production, being possible to predict that the presence of weeds would be correlated with the soil quality of urban homegardens.

Methods

Study sites

We conducted the study in 30 urban homegardens in 17 districts in the municipality of Alta Floresta, Mato Grosso State, Brazil (Fig. 1). Alta Floresta is a municipality that has many urban homegardens rele-vant for the conservation of agrobiodiversity and food security. It offers a huge agricultural potential. The 30 homegardens were selected by consulting the most promising ones according to the local secretary of agriculture; here, the following criteria were consid-ered: people’s willingness to participate in the survey; species diversity; and production for food and sale. Alta Floresta is located in the Southern Amazon (09°520₃₂00 _{and 56°05}0₁₀00_{), at an altitude of about}

260 m. The climate is tropical with two well-defined seasons: rainy summer and dry winter. The

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temperatures range between 20.6 and 31°C, with average of 24.8 °C and precipitation between 2000 and 2030 mm (Seplan 2001). The predominant soil types are dystrophic yellow agrosols and red-yellow latosols (Embrapa 2013). The municipality presents flat topographic areas (with up to 3% of slope), corresponding to 30% of the total area; slightly flat topography (3–8% slope), occupying area greater than 55%; wavy topography (8–20% of slope), occupying 10%; and mountainous area (above 45–75% of slope), occupying about 5% of the municipality.

Evaluation of weeds

We sampled 30 urban homegardens to assess the weed composition and quantitative descriptors. From each urban homegarden, we collected five samples, totaling 150 weed samples. The sampling point setting followed the shape of a ‘‘W,’’ with one sample at

each end (see Medeiros and Steiner2002; Mulugeta and Stoltenberg1997). The distance from one end to the other depended on the size of the respective homegarden (20 m average).

In order to obtain weed seed bank samples (Costa-longa 2006), we used a rectangular wood template with a length of 30 cm, a width of 20 cm, and a depth of 5 cm. We only sampled the upper 5-cm soil layer as this usually contains the highest concentration of weed seeds (Pelissari et al.2013).

After the random release of the wooden jig, we removed the weeds by hand. If the wood template fell into place with a stone or tree, for example, we moved to the right of the sampled point. We then removed the organic litter with a spatula to collect the weed seed bank samples and placed it into black plastic bags. The bags were labeled and transported to the greenhouse. For germination, we placed the samples in aluminum trays in the greenhouse on a workbench with a height of 1.0 m; samples were arranged randomly. Manual Fig. 1 Municipality of Alta Floresta, Mato Grosso State, Brazil, and urban homegardens surveyed in this study

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irrigation was performed once a day to maintain adequate soil moisture for germination and plant development (Fig.2).

Over a period of 6 months, we counted and identified germinated seedlings every 30 days. Emerged seedlings were photographed, identified, quantified, and removed from the trays. Seedling identification was performed with the help of literature (e.g., Lorenzi2008).

Soil fertility evaluation

For the analysis of the physical and chemical param-eters of the soil, we collected samples at a depth of 0–10 cm of the soil next to the place where the samples of the weeds seed bank were collected. The five samples were homogenized to obtain one sample per urban homegarden. We measured the following soil variables: pH, phosphorus, potassium, calcium, magnesium, aluminum, hydrogen, organic matter, sum basis, cation exchange capacity, base saturation, and soil texture using the methods indicated by Ribeiro et al. (1999). For the interpretation of the soil analysis of urban homegarden, the parameters described by Alvarez et al. (1999), Kaca´lek et al.

(2009), Flores-Delgadillo et al. (2011), and Kumar (2016) were used.

In addition to the physical and chemical evaluation of the soil, an investigation was conducted on how each homegardener evaluates soil fertility of the garden by means of a semi-structured interview (Garrote 2004). This interview was focused on how homegardeners identify whether a soil is fertile (‘‘good’’) or infertile (‘‘bad’’) in each homegarden.

Data analysis

The quantitative parameters used to assess the struc-ture of weed species were density, frequency, and dominance (both absolute and relative values), and Importance Value Index—IVI (Pitelli and Bianco

2013) according to the following formula: IVI = RD ? RF ? RD, where RD = relative density, RF = relative frequency, and RD = relative domi-nance. We also obtained Shannon’s diversity and Pielou’s evenness indexes.

We performed an ordination analysis (detrended correspondence analysis, DCA; Hill and Gauch1980) to evaluate variations in the suite of emerging weed species from the seed bank among all 30 urban

Fig. 2 Soil samples of the urban homegardens of the city of Alta Floresta, Mato Grosso State, Brazil, in aluminum trays for the identification of weeds

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homegardens, using a matrix with the occurrence records in each homegarden (except species that occurred only once) in PC-ORD for Windows— Multivariate Analysis of Ecological Data—version 6.25 (McCune and Mefford 2011). We marked the options of ‘Downweight rare species’ and ‘Rescale axis’ of PC-ORD. This step was followed by a modeling of the influence of soil variables on floristic variation of weeds by Redundancy Analysis (RDA). Here, we used the same community response matrix mentioned above, which was Hellinger-transformed to meet the recommendations by Legendre and Gal-lagher (2001). Prior to modeling, we used Principal Components Analysis (PCA) to condense the infor-mation on soil variables in a smaller group of new dimensional compositions and, thus, remove collinearities (McGarigal et al. 2000). In this step, for each group of collinear variables, we retained only the variable that was most correlated with the respec-tive PCA axis. For example, base saturation (V %) and aluminum saturation (m %) were highly collinear (rPearson[ 0.9) in the PCA Axis 1; because V % was most correlated to this axis, the aluminum saturation was removed from the analysis. We ended up this procedure with nine soil variables: hydrogen (H), potential of hydrogen (pH), base saturation (V %), aluminum (Al), organic matter (OM), phosphorus (P), calcium (Ca), magnesium (Mg), and potassium (K). With this set of non-collinear variables, we used the ‘packfor’ package (Dray et al.2011) of R version 3.3.2 (R Core Team2016) to perform the forward selection (Blanchet et al.2008) of the RDA models. Spatial data were obtained based on the method of Moran’s Eigenvector Maps (MEMs), which represent the spatiality of the data at different scales (Dray et al.

2006); MEMs were also forward-selected. Subse-quently, we prepared environmental and spatial mod-els under RDA to evaluate the potential influence of soil variables on the floristic variation by means of a variance partitioning framework (Peres-Neto et al.

2006). Here, the variance explained by RDA was partitioned between environmental (soil) and spatial (MEMs) components, and each ‘‘pure’’ fraction was tested by ANOVA based on permutations.

Results and discussion

Chemical characterization of urban homegardens soil

The pH values of the homegardens soils ranged from 5.1 to 7.6 (Table 1). According to the agronomic classification, values above 4.5 are considered low acidity (Alvarez et al. 1999). These values are common for Amazonian soils (Fearnside and Leal Filho 2001). Soil properties of forest ecosystems depend on synergy of both parent material and organisms living in the soil, i.e., tree species commu-nities including related plant and animal species (Kaca´lek et al. 2009). The use of crop manure and ashes in the management of urban homegardens may be responsible for the high pH values of these soils. Soil fertility improvement of urban homegardens can be attributed mainly to the deposition of residues around dwellings, although further investigation is needed on the role of trees in accessing nutrient pools at greater soil depths (Pinho et al.2011).

Base saturation values (V %) ranged from 36.24 to 87.49%; three urban homegardens soils had low levels (20.1–40%) and the other 27 soils showed average levels (above 40.1%), which are considered good/very good for agronomic purposes (Alvarez et al. 1999). For most crops, recommended levels of soil saturation range from 60 to 80% (Motta and Lima2006).

Calcium and magnesium concentrations ranged from 1.10 to 6.44 cmolcdm-3and from 0.29 to 1.49 cmolc dm-3, respectively. Only one urban homegar-den showed low calcium levels, while the other 29 had average levels (good and very good for agronomic purposes; Alvarez et al. 1999). In most urban home-gardens, magnesium showed values from 0.42 to 1.49 cmolc dm-3, which are classified as average/good-quality (Alvarez et al.1999). Only four urban home-gardens had low magnesium levels (below 0.45 cmolc dm-3). Potassium values ranged from 25 to 100 mg dm-3. Values below 40 mg dm-3 are con-sidered low (Alvarez et al.1999) and were observed for 10 urban homegardens. In most gardens, values varied from 41 to 70 mg dm-3. Phosphorus values ranged from 1.45 to 314.53 mg dm-3, with a large variation. Ten urban homegardens had values above 18 mg dm-3, which are classified as very good (Alvarez et al.1999), while three urban homegardens had values considered good. However, most urban

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homegardens (17) had values classified as low. This result was expected, since the soils in the Amazon region are not considered high in P (Fearnside and Leal Filho2001).

Organic matter values ranged between 10.67 and 40.97 g dm-1. Half of the sampled urban homegar-dens are classified as low as it presented values below 20 g dm-1 (Alvarez et al. 1999; Flores-Delgadillo et al. 2011). On the other hand, we observed values

from 20.10 to 40 g dm-1, classified as average (Alvarez et al. 1999). The high levels observed for organic matter may be partially due to nutrient uptake by deep-rooting trees and subsequent nutrient cycling in the form of leaf litter, fruits, and branches (Pinho et al.2011). Tree growing in combination to agricul-ture (agroforestry systems) as well as numerous vegetation management regimes in cultural land-scapes may improve nutrient availability and Table 1 Chemical characteristics of the soils of urban agroforestry homegardens in the 0–10 cm layer in Alta Floresta—MT, Southern Amazon, Brazil

Homegarden (HG) pH P K? Ca2? Mg2? Al3? H? O. M. V CEC (H2O) mg dm-3 cmolcdm-3 g dm-3 % cmolcdm-3 HG01 6.80 314.53 100.00 6.44 0.87 0.00 1.40 24.77 84.39 8.89 HG02 5.60 18.46 62.00 3.10 0.77 0.00 2.80 24.77 58.96 6.82 HG03 5.70 7.95 28.00 2.85 0.45 0.00 2.00 17.05 62.69 5.36 HG04 6.00 7.67 41.00 2.44 0.68 0.00 2.00 21.10 61.78 5.23 HG05 5.60 6.19 40.00 1.37 0.32 0.10 1.80 15.33 48.47 3.69 HG06 5.40 4.70 37.29 30.00 0.37 0.20 2.30 15.94 46.59 4.68 HG07 5.00 4.40 29.00 1.10 0.33 0.40 2.10 13.63 37.50 4.00 HG08 6.20 36.23 51.00 3.96 0.74 0.00 1.60 22.39 75.12 6.43 HG09 6.00 6.33 61.00 2.19 0.54 0.00 1.80 16.31 61.67 4.70 HG10 5.40 5.91 57.00 2.75 0.51 0.20 3.40 24.50 48.62 7.01 HG11 7.10 4.85 64.00 4.46 1.49 0.00 1.10 20.46 84.73 7.21 HG12 6.00 15.62 76.00 2.54 0.48 0.00 2.10 18.68 60.46 5.31 HG13 6.30 20.17 71.00 5.19 1.13 0.00 1.40 40.97 82.27 7.90 HG14 6.20 19.21 57.00 2.58 0.82 0.00 1.70 20.33 67.58 5.24 HG15 5.60 3.79 39.00 1.96 0.46 0.05 1.95 16.06 55.74 4.52 HG16 5.80 3.55 25.00 3.02 0.67 0.00 2.00 22.26 65.19 5.75 HG17 5.60 3.49 50.00 1.98 0.47 0.10 2.40 18.93 50.79 5.08 HG18 5.80 1.45 80.00 2.35 0.69 0.00 1.60 20.59 67.00 4.85 HG19 5.30 27.25 71.00 2.10 0.98 0.10 2.80 22.26 52.88 6.15 HG20 7.60 25.95 40.00 5.21 0.29 0.00 0.80 15.70 87.49 6.40 HG21 5.20 8.89 52.00 1.71 0.45 0.10 3.53 15.33 38.66 5.92 HG22 5.10 23.56 38.00 1.92 0.42 0.20 4.09 16.06 36.24 6.73 HG23 6.30 8.22 59.00 6.79 1.39 0.00 3.63 35.25 69.64 11.96 HG24 5.90 4.59 44.00 3.81 1.21 0.00 3.30 18.68 60.88 8.44 HG25 6.80 43.46 89.00 9.44 2.92 0.00 3.63 37.92 77.62 16.22 HG26 6.30 17.97 80.00 5.71 1.23 0.00 4.13 28.28 63.40 11.28 HG27 6.00 6.14 55.00 4.08 0.87 0.00 2.64 19.18 65.83 7.72 HG28 6.10 12.29 25.00 2.30 0.72 0.00 1.82 10.67 62.90 4.91 HG29 6.40 6.49 74.00 4.38 1.02 0.00 2.48 20.71 69.30 8.08 HG30 6.90 3.67 37.00 4.36 0.77 0.00 1.32 12.91 79.82 6.54 P phosphorous, K?potassium, Ca2?calcium, Mg2?magnesium, Al3?aluminum, H?hydrogen, O. M. organic matter, v % base saturation, CEC cation exchange capacity

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efficiency of use, and may reduce erosion (Kumar

2016). Trees that do not fix nitrogen can improve the physical, chemical, and biological characteristics of soils by adding a significant amount of organic matter, besides the recycling of nutrients (Jose 2009). The high yield of the homegarden is maintained without applying industrial fertilizers (Jensen 1993). This author concludes that the sustainability of homegar-dens is linked to a medium fertile soil with nutrient reserves due to sustainable practices such as the use of organic fertilizers. Besides the use of organic fertiliz-ers, plant biomass protects the soil from erosion and its drying, and the high diversity of plant species in homegardens allows different root strata to access deeper nutrients, thereby contributing to the nutri-tional supply of plant species that comprise the crops. In turn, homegardeners identify the soil as being fertile (‘‘good’’) or infertile (‘‘bad’’) from color, texture, presence or absence of ants, productivity, and the presence or absence of spontaneous plants. A fertile soil for the homegardeners is one in which the coloration is black, with a large amount of organic material and organisms such as earthworms and termites. The soil color described by the homegar-deners may be a more useful tool for researchers and extensionists to effectively dialogue and work with such homegardeners to understand their natural char-acteristics and to develop improved crop production systems (Saito et al. 2006). The homegardeners use spontaneous plants as indicators of soil quality of their homegardens. They reported that occurrence of the species Commelina diffusa Burm.f. indicates fertile soil. Altieri and Nicholls (2002) argue that for many homegardeners the presence of earthworms on the ground means living soil and the intense green color of leaves reflect a good nutritional status of the plants. The ethno-knowledge on the soil is consistent with the findings of Rinklin (1992). The traditional knowledge has shown that the homegarden system is ideally suited for regions characterized by highly weathered soils with relatively lower nutrient endowments (Ku-mar2006).

The classification of soils by the homegardeners showed a correlation with the chemical analysis of the soil since soil of the majority of the urban homegar-dens studied presented levels above the desired minimum of macronutrients, pH and saturation by bases as presented previously. This result is in agreement with that of Saito et al. (2006), who

reported that homegardeners describe their soils in a holistic view, relating the occurrence of weeds to the soil with their quality. Homegardeners are knowl-edgeable about their soils and their knowledge corre-lates well with the scientific understanding of soils (Saito et al.2006).

Weeds in the soil seed bank of urban homegardens

In the soil weed seed bank, we found 5652 individuals belonging to 70 species and 27 families. However, in the soil samples from six urban homegardens, we did not observe seed germination. Total weeds density was 812 plants m-2. Relative density is the parameter that most contributes to the importance of a species in a given area (Balduı´no et al. 2005). The Shannon– Wiener index of species diversity was 2.83 and Pielou evenness was 0.682. These are below values usually found in natural ecosystems. Gliessman (2001), for instance, pointed out that natural ecosystems that are relatively diversified have a Shannon–Wiener diver-sity index between 3 and 4. This is in line with the fact that weeds are dominant in agricultural environments and can be indicators of a particular soil conditions, climate, and historical agricultural practices (Pitelli and Bianco2013).

Poaceae was the family with the largest number of individuals (2171), followed by Cyperaceae (1171) (Table 2). Several species of weeds were common in most of the studied urban homegardens, such as Kyllinga brevifolia Rottb, Chloris gayana Kunth, Stemodia verticillata (Mill.) Hassl., Cyperus rotundus L., Hyptis atrorubens Poit., and Andropogon leu-costachyus Kunth. In a study by Silva (2007), Poaceae totaled 37% of the most important weeds in productive agrosystems. These weed species are common to the urban homegardens because of the similar character-istics of the soils.

Poaceae showed the highest relative density (38.42%) and the highest IVI (59.88%), followed by Cyperaceae (20.72 and 33.68%, respectively). High IVI values indicate high occurrence, distribution, and a capacity to dominate the cultivated plants in cropping systems (Nascimento 2010). Despite the high weed diversity in the sampled urban homegar-dens, the dominance of Poaceae and Cyperaceae in such different areas emphasizes the plasticity and the hardiness of species of these families. Several species of the Poaceae family are perennial and produce a

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large quantity of seeds that spread into nearby environments (Holm et al.1991). Data from previous urban homegardens surveys have demonstrated that the presence of weeds, especially from the Cyperaceae family, holds a prominent position in most of the quantitative parameters evaluated, which is closely related to the aggressiveness of the members of this family (Silva et al.2013). According to Carvalho and Pitelli (1992), using the same soil management system for several consecutive years can modify the vegeta-tive flora and change the size and the composition of the soils seed bank. Thus, the large number of individuals of the family Cyperaceae in urban home-gardens can be explained by the seed bank containing a higher number of seeds in relation to other species (Silva et al.2013). Understanding the distribution of weeds in urban homegardens is therefore crucial for

the development of efficient control methods to sustainably manage the volunteer plants in Amazonian urban homegardens.

Variation in the composition of weeds and potential drivers in urban homegardens

The length of the gradient of the DCA (Fig.3) was 2.81 in the first axis and 1.93 in the second, showing that more than half of the species had been overlapped along the primary gradient (Hill and Gauch 1980). This species replacement could basically be associated with environmental factors, the way of handling homegardeners, the successional stage of the urban homegardens, and/or the intrinsic conditions of the weeds (see Pitelli and Bianco2013). The understand-ing of spatial distribution of weeds can inform Table 2 Quantitative

parameters (%) of the families of weeds from the soil seed bank of the Amazonian urban homegardens investigated in this study

NInd number of individuals, AbsDe absolute density, RelDe relative density, NAm number of samples in which the family is present, AbsFr absolute frequency, RelFr relative frequency, IVI index of importance value, NSpp number of species

Family NInd AbsDe RelDe NAm AbsFr RelFr IVI NSpp %Spp Poaceae 2171 311.9 38.41 111 95.69 21.47 59.88 13 18.57 Cyperaceae 1171 168.2 20.72 67 57.76 12.96 33.68 3 4.29 Phyllanthaceae 163 23.4 2.88 53 45.69 10.25 13.14 1 1.43 Commelinaceae 219 31.5 3.87 43 37.07 8.32 12.19 3 4.29 Lamiaceae 359 51.6 6.35 30 25.86 5.80 12.15 2 2.86 Molluginaceae 249 35.8 4.41 35 30.17 6.77 11.18 1 1.43 Rubiaceae 129 18.5 2.28 36 31.03 6.96 9.25 2 2.86 Plantaginaceae 404 58.0 7.15 6 5.17 1.16 8.31 1 1.43 Asteraceae 183 26.3 3.24 26 22.41 5.03 8.27 9 12.86 Amaranthaceae 67 9.6 1.19 18 15.52 3.48 4.67 1 1.43 Loganiaceae 89 12.8 1.57 12 10.34 2.32 3.90 1 1.43 Pontederiaceae 167 24.0 2.95 2 1.72 0.39 3.34 1 1.43 Euphorbiaceae 26 3.7 0.46 14 12.07 2.71 3.17 2 2.86 Caryophyllaceae 52 7.5 0.92 9 7.76 1.74 2.66 1 1.43 Portulacaceae 32 4.6 0.57 10 8.62 1.93 2.50 3 4.29 Onagraceae 28 4.0 0.50 7 6.03 1.35 1.85 3 4.29 Fabaceae 33 4.7 0.58 6 5.17 1.16 1.74 3 4.29 Brassicaceae 41 5.9 0.73 5 4.31 0.97 1.69 1 1.43 Malvaceae 22 3.2 0.39 5 4.31 0.97 1.36 5 7.14 Solanaceae 18 2.6 0.32 5 4.31 0.97 1.29 5 7.14 Apiaceae 9 1.3 0.16 5 4.31 0.97 1.13 1 1.43 Verbenaceae 5 0.7 0.09 4 3.45 0.77 0.86 2 2.86 Chenopodiaceae 4 0.6 0.07 3 2.59 0.58 0.65 1 1.43 Caricaceae 3 0.4 0.05 1 0.86 0.19 0.25 1 1.43 Balsaminaceae 3 0.4 0.05 1 0.86 0.19 0.25 1 1.43 Lythraceae 2 0.3 0.04 1 0.86 0.19 0.23 1 1.43 Myrtaceae 2 0.3 0.04 1 0.86 0.19 0.23 1 1.43

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decision-makers with respect to the control of these species, mainly in urban homegardens, where the use of pesticides, for instance, should be avoided due to the proximity to residences.

Considering the possible drivers of variation in species composition, the partition of the set of variables (Fig. 4) indicated that the distribution of species in the urban homegardens can hardly be explained by environmental and spatial parameters as they were not statistically significant (p [ 0.05). The

variables that were not measured (residuals) repre-sented 96% of the data variation. Thus, it is not possible to conclude that the presence of weeds is associated with variations in geographic space or chemical soil quality. This result may have been influenced by the fact that we used a composite sample for each urban homegarden. However, the results are interesting because they suggest correlations that can be addressed in further studies, particularly highlight-ing the role of the elements on the diversity of the population of some weed species. If we had collected soil variables from each of five sampling points separately, we could have obtained models at a finer scale. This issue may be addressed in further studies. The floristic variation of weeds could be related to microclimate conditions such as humidity (Ba´tori et al. 2014). In urban homegardens, humidity is generally high, since the cultivation of fruit-bearing forest species in the gardens provides a shaded environment. Such condition encourages the coexis-tence of many species of weeds (Anzalone et al.2012). Another possible association is the management of the condition of the gardens that differ according to the culture of each garden and depend on agricultural methods. We suggest that these variables are consid-ered in works that seek to model the species compo-sition in urban homegardens.

Fig. 3 Detrended Correspondence Analysis (DCA) demon-strating the floristic relationships among the species found in 30 urban homegardens studied in Alta Floresta, Southern Amazon, Brazil (except species that occurred only once). Triangles refer

to urban homegardens. The name of each species is shown in eight characters, the first four referring to the genus and the last four referring to the specific epithet. Full names of species can be found in Supplementary Material 1

Fig. 4 Variance partitioning among components used to model weed species variation in 30 urban homegardens studied in Alta Floresta, Southern Amazon, Brazil. Both environmental and spatial ‘‘pure’’ fractions were not statistically significant (p [ 0.05)

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In summary, the soil of most of the studied urban homegardens had levels above the desired minimum of macronutrients, pH and base saturation. Agro-forestry systems provide organic matter and nutrient cycling. In addition, the soil of homegardens is usually well managed, probably because they are in close proximity to the residences. Improved soil fertility in urban homegardens can be attributed mainly to the deposition of residues around dwellings, although further research is needed on the role of trees in accessing nutrient pools at greater soil depths. The evaluation of soil quality in urban homegardens by means of certain attributes is quite complex due to the large number of definitions of a soil with quality for a particular use; in addition, there is a multiplicity of interrelations between physical, chemical, and bio-logical factors that control the processes and patterns related to the variation of soil in time and space (Melloni et al.2008).

However, based on the correlation between the presence of weeds and the physical–chemical quality of the soil, we must recognize that we are still far from obtaining a satisfactory model that can explain the composition of weeds and their relation with soil fertility in urban homegardens. More elaborate studies are needed with a greater number of urban homegar-dens with different soil conditions to explore the relationship of soil fertility to weed species. Such future studies could assess the importance of agro-forestry systems to maintain chemical soil quality. In addition, different patterns of weed species composi-tion should be compared in terms of soil characteris-tics to develop compositional models of weed species indicative of different soil management situations.

Acknowledgements The first author is indebted to CAPES (Coordenaçaõ para Aperfeiçoamento de Pessoal de Nı´vel Superior) the Brazilian Government Agency, that provided a Masters scholarship.

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