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Introduction
Composting is the natural process of aerobic decomposition of fermentable organic matter by the action of microorganisms that carry out reactions of mineralization and partial humification of organic substances (Trejo-Vázquez, 1994). In Mexico City (CDMX), where 20.4 % of the urban soil is covered by green areas (Instituto Nacional de Ecología [INE], 2003), green waste (solid organic waste from tree pruning and lawn mowing) is commonly used in compost production for its high production and easy handling and collection (INE, 2007a).
Applying compost as a soil improver is a common practice in the agricultural and livestock sectors (Bernal, Alburquerque, & Moral, 2009; Larney & Hao, 2007), where its large-scale production is based on composting farm animal manure or excreta. It is widely held that the main contributions of compost to the soil are organic matter and nutrients. Moreover, it improves soil aeration, moisture retention, structure (prevents erosion), and physicochemical composition, as well as the development of beneficial microorganisms, and absorption of solar rays; it can also increase the soil temperature in the cold season and reduce or eliminate the need for chemical fertilizers (Bernal et al., 2009; Larney & Hao, 2007).
There are multiple studies on the effect of using improvers and compost on agricultural soils. Evanylo et al. (2008) applied poultry waste directly to farmland and observed improvements in bulk density and porosity of soils, as well as a decrease in N and P losses due to leaching during runoff. Chang, Chung, and Tsai (2007) used compost for three years on 24 crops, under greenhouse conditions and under subtropical conditions, and showed that its use provided the soil with a greater amount of total nitrogen (TN) and organic matter (OM) than fertilizers. On the other hand, Widman-Aguayo, Herrera-Rodríguez, and Cabañas- Vargas (2005) noted that urban solid waste (USW) compost can be used to improve the characteristics of poor agricultural soils. It has been observed that composted garden waste eliminates some diseases caused by Rhizoctonia (Kuter, Hoitink, & Chen, 1988).
Environmental standard NADF-006-RNAT-2012 (Gobierno del Distrito Federal, 2013) defines Mexico City’s green areas as "any surface covered with vegetation, natural or induced …” It also states that these areas contribute in a fundamental way to improve the quality of life of the inhabitants and are indispensable for reducing heat islands, capturing pollutants and suspended particles, producing oxygen, stopping soil erosion, increasing humidity, decreasing noise levels and capturing rainwater, as well as being places of refuge and food for various forms of life.
Green areas are also related to public health and recreation, they enhance the urban image and they generate positive effects on mental health, so it is of great interest for the city government to protect and promote them.
In spite of the above, some green areas of Mexico City have been established in land with soil altered with construction or other residues. Although there is an inventory of these areas in Mexico City, it only indicates the area that they cover and their quality or conditions are unknown.
Soils must be able to absorb, retain and supply water to the plants they sustain. Today, much of the soil in Mexico City’s urban area is unable to fully carry out these functions, due to the variety of materials that are used to fill planters, parks and other green areas. On the other hand, changing plants based on the season (every three to four months) is an increasingly recurrent practice in the city. All of the above actions can cause nutrient deficiency and reduced water holding capacity (WHC, due to the content of sands and coarse materials), among other effects.
The work undertaken by the Mexico City government to mitigate the problems mentioned consists of applying compost made mainly with green waste. Environmental standard NADF-020-AMBT-2011 (Gobierno del Distrito Federal, 2012) establishes three types of compost based on the quality and uses that can be ascribed to it. Type A is used as a substrate in nurseries and as a soil substitute for pots, B is used in organic farming and reforestation and C is applied in landscape, urban green areas and reforestation.
The aim of this study was to evaluate the effect of adding compost, produced with green waste, to a non-natural soil, in terms of its organic matter content and water holding capacity. This was done in order to demonstrate that this type of compost can be an effective alternative to improve the quality of altered soils. To evaluate its effect, beans and maize were used as representative plants of two fast-growing plant species. Thus, indirectly, the results of this study can indicate whether the application of compost to filled and altered soils can improve soil conditions to favor the growth of ornamental plants, used in urban areas for revitalization purposes.
Materials and methods
Compost production
The compost used was made at the National Polytechnic Institute (IPN) Compost Plant from green waste. The raw materials used were: 6,000 kg of grass, 1,133 kg of leaf, 1,866 kg of mulch and 3,200 kg of mature compost (inoculum). Two intercalated layers of each material were placed to form the pile of material to be composted. The approximate initial volume of the compost was 35 m3 (11 x 1.6 x 2 m). The pile was manually turned over for three months (13 turns); later, the maturation period was two months, giving a total of five months. During the process the temperature (T), pH and moisture (M) were monitored.
Table 1 shows the values of the physicochemical parameters, both of the compost and soil used. To determine the quality of the compost, some of its values were compared with those indicated in specialized bibliography and in current regulations (NADF-020-AMBT-2011 [Gobierno del Distrito Federal, 2012] and NTEA-006-SMA-RS-2006 [Gobierno del Estado de México, 2006]).
Table 1 Physico-chemical parameter values of compost and soil.
| Parameter | Compost | Soil |
|---|---|---|
| pH | 8.076 ± 0.1 | 8.035 ± 0.075 |
| OM* (%) | 25.117 ± 0.42 | 0.135 ± 0.001 |
| TN | 0.95 ± 0.006 | 0.088 ± 0.005 |
| Organic carbon (%) | 14.57 ± 0.016 | 0.078 ± 0.001 |
| C/N Ratio | 15.42 ± 0.21 | - |
| T (°C final) | ≤10 | - |
| Bd (g·cm-3) | 0.46 ± 0.02 | 1.31 ± 0.005 |
| Pd (g·cm-3) | 0.91 | 2.00 |
| PS (%) | 50.7 ± 0.075 | 34.4 ± 0.26 |
| M (%) | 5.06 ± 0.102 | 2.51 ± 0.14 |
| FC (%) | - | 31.3 ± 0.13 |
| PWP (%) | - | 2.18 ± 0.24 |
*OM = organic matter, TN = total nitrogen, C = carbon, C/N = carbon/nitrogen ratio, T = temperature, Bd = bulk density, Pd = particle density, PS = porous space, M = moisture, FC = field capacity and PWP = permanent wilting point.
Characteristics of the compost used
According to NADF-020-AMBT-2011 (Gobierno del Distrito Federal, 2012), IPN compost can be considered as type B and C for pH (8.076 ± 0.1), type C for the OM percentage (25.117 ± 0.42) and type A in relation to C/N (15.42 ± 0.21) and T (≤ 10 ° C), while for NTEA-006- SMA-RS-2006 (Gobierno del Estado de México, 2006), the values are within the established limits, with the exception of the C/N ratio that presents high values, giving a moderately alkaline end product. Navarro (2002) recommends a C/N ratio lower than 12. Bernal et al. (2009) mention that, like manure, grass residues have a high N concentration, giving a very low C/N ratio and, in some cases, great alkalinity.
Obtaining of the soil used
The soil used was obtained from the green areas at IPN’s Professional Interdisciplinary Biotechnology Unit ((UPIBI-IPN), and is mainly composed of the natural soil of the area, gravel and sand. The soil of this land was chosen because it presents conditions similar to those of many green areas in Mexico City. To prepare the treatments, the soil was conditioned by removing large-sized stones and material, and it was aerated and exposed to the sun in order to eliminate larvae and insects.
Characteristics of the soil used
Based on NOM-021-SEMARNAT-2000 (Secretaría de Medio Ambiente y Recursos Naturales [SEMARNAT], 2002), the soil used is classified as moderately alkaline, non-volcanic, with very low OM content and low TN content, clayey loam texture (predominantly clay) and bulk density (Bd) of 1.312 ± 0.005 g·cm-3. The M % presented by the soil was 2.51 ± 0.14, a value related to its low OM content, which indicates that it requires the addition of more water, to the level that it can withstand, to meet the requirements of the plants.
Experimental design
Six treatments were used: three of them were mixtures of soil and compost in different proportions (w/w: 10, 20 and 30 % for T1, T2 and T3 respectively); soil with chemical fertilizer added (T4); 18- 46-00 Triple; a negative control with 100 % soil (T0) and a positive control with 100 % compost (T5). The proportions (soil-compost) used are common for greenhouse planting and the fertilizer rate was as indicated on the safety sheets of the product used. Subsequently, maize (Zea mays L.) and bean (Phaseolus vulgaris L.) were sown in each treatment. For each treatment and species, seven replicates were prepared. The plants were left to develop until reaching permanent wilting. Additionally, in the same period of time, samples of the different treatments were taken for analysis.
Sampling
Compost samples were obtained by taking subsamples from the pile, at equidistant points, horizontally and vertically, taking care that all heights and depths had the same distance. The subsamples were homogenized and the final sample (approximately
1 kg) was obtained with the quartering technique. For taking samples from the soil, soil-compost mixtures and the soil-fertilizer combination, the quartering technique was followed. Samples of the mixtures were obtained prior to sowing and after permanent wilting of the plants.
Laboratory analysis
The parameters pH, TN, OM (%), M (%) and Bd (g·cm-3) were evaluated using the AS-02, AS-25, AS-07, AS-05 and AS-03 methods, respectively (NOM-021-SEMARNAT-2000 [SEMARNAT, 2002]). The C/N ratio (obtained from the relationship between OM and TN contents) was determined using the method of Jackson (1970), field capacity (FC, %) and permanent wilting point (PWP, %) were obtained with the method recommended by Rodríguez- Fuentes and Rodríguez-Absi (2002) and particle density (Pd, g·cm-3) was obtained with the procedure suggested by Rodríguez-Fuentes and Rodríguez-Absi (2002). Pore space (PS, %), which represents the portion of the soil occupied by air and water, was determined from the Bd and Pd data obtained (Equation 1).
Determination of WHC
Soil water properties are an important component of soil characteristics. The capacity of a soil to supply water can be visually measured by the existence and productivity of the plant species it supports. Theoretically, the determination of WHC or useable water is the result of the difference between FC and temporary or permanent wilting point; obtaining it requires the application of models with physical bases that estimate the amount of water contained in a soil at its FC under natural conditions (Domingo- Santos, Fernández, Corral-Pazos, & Rapp-Arrarás, 2006). The calculation of field WHC uses FC, PWP and Bd data, and considers the percentage of rock fragments. This paper uses Equation 2, which represents the calculation method of the National Soil Survey Handbook (NSSH) and considers the percentage of rock fragments (1-F) = 1 (United States Department of Agriculture [USDA], 2010).
Where WHC is expressed in mm of water, W1/3 is the weight percentage of water retained at 1/3-bar tension (g water·100 g-1 soil < 2 mm), W15 is the weight percentage of water retained at 15-bar tension (g water·100 g-1 soil < 2mm), e is the thickness (cm), f is the rock fragment conversion factor derived from: volume moist <2 mm fabric (cm3) by volume moist whole soil (cm3), a factor that applies to field measurements, if there were no coarse fragments: (1-f) = 1 cm.
Results and discussion
Soil-compost and soil-fertilizer treatments
The physicochemical analyses show that before sowing the most alkaline pH corresponded to T4, while T1, T2 and T3 presented no effect with respect to T0. This contrasts with the findings reported by Casado-Vela et al. (2006), who obtained a decrease in pH as the composted sewage sludge dose increased. For their part, Chang et al. (2007) reported a decrease in pH when using compost compared to fertilizers. Olivares- Campos, Hernández-Rodríguez, Vences-Contreras, Jáquez-Balderrama, and Ojeda-Barrios (2012) observed no differences in pH due to the addition of compost, vermicompost and fertilizers. Table 2 presents the data obtained for pH, OM, TN, C and C/N ratio, as well as their respective standard errors.
Table 2 Chemical parameter values of all treatments tested.
| Treatment | pH | OM* (%) | TN | C | C | |
|---|---|---|---|---|---|---|
| T0 | A | 8.035 ± 0.075 | 1.97 ± 0.019 | 0.088 ± 0.006 | 1.14 ± 0.011 | 12.97 ± 0.802 |
| B | 7.975 ± 0.495 | 3.85 ± 0.212 | 0.171 ± 0.006 | 2.23 ± 0.123 | 13.03 ± 0.969 | |
| C | 8.45 ± 0.2 | 4.74 ± 0.194 | 0.41 ± 0.079 | 2.75 ± 0.112 | 6.78 ± 1.776 | |
| T1 | A | 8 ± 0.05 | 5.42 ± 0.096 | 0.086 ± 0.003 | 3.14 ± 0.056 | 36.55 ± 1.634 |
| B | 7.88 ± 0.3 | 6.75 ± 0.206 | 0.48 ± 0.093 | 3.91 ± 0.120 | 8.16 ± 1.345 | |
| C | 8.2 ± 0.07 | 8.08 ± 0.060 | 0.29 ± 0.002 | 4.69 ± 0.035 | 16.28 ± 0.225 | |
| T2 | A | 8.03 ± 0.04 | 5.54 ± 0.355 | 0.266 ± 0.002 | 3.21 ± 0.206 | 12.10 ± 0.699 |
| B | 8.09 ± 0.09 | 8.23 ± 0.270 | 0.555 ± 0.101 | 4.77 ± 0.157 | 8.59 ± 1.704 | |
| C | 8.1 ± 0.1 | 10.20 ± 0.389 | 0.497 ± 0.000 | 5.91 ± 0.226 | 11.89 ± 0.453 | |
| T3 | A | 8.03 ± 0.04 | 5.57 ± 0.092 | 0.244 ± 0.005 | 3.23 ± 0.053 | 13.21 ± 0.149 |
| B | 7.96 ± 0.02 | 9.45 ± 0.096 | 0.455 ± 0.100 | 5.48 ± 0.056 | 12.03 ± 3.476 | |
| C | 8 | 9.31 ± 0.391 | 0.489 ± 0.004 | 5.40 ± 0.227 | 11.06 ± 0.513 | |
| T4 | A | 7.51 ± 0.02 | 1.99 ± 0.004 | 0.304 ± 0.070 | 1.15 ± 0.002 | 3.80 ± 0.787 |
| B | 8.08 ± 0.24 | 5.61 ± 0.326 | 0.295 ± 0.051 | 3.25 ± 0.189 | 11.02 ± 1.189 | |
| C | 8.25 ± 0.09 | 4.41 ± 0.303 | 0.396 ± 0.001 | 2.56 ± 0.176 | 6.47 ± 0.425 | |
| T5 | A | 8.1 ± 0.1 | 25.12 ± 0.420 | 0.945 ± 0.006 | 14.57 ± 0.244 | 15.42 ± 0.209 |
| B | 7.55 ± 0.05 | 7.89 ± 0.846 | 0.753 ± 0.002 | 4.57 ± 0.491 | 6.07 ± 0.669 | |
| C | 7.97 ± 0.02 | 8.74 ± 0.098 | 0.845 ± 0.12 | 5.070 ± 0.057 | 6.00 ± 1.009 | |
*OM = organic matter, TN = total nitrogen, C = carbon, C/N = carbon/nitrogen ratio, T0 = negative control (100 % soil), T1 = soil plus 10 % compost, T2 = soil plus 20 % compost, T3 = soil plus 30 % compost, T4 = soil with chemical fertilizer, T5 = positive control (100 % compost), A = analysis before sowing, B = analysis after sowing and C = analysis after sowing beans.
Generally, soil pH ranges from 4.5 to 9.0. Soils with pH > 7.5 may present phosphorus availability problems. However, a pH of 8.5 is used as the limit, since higher values could restrict some soil functions. It is known that OM can help buffer against pH changes (Chang et al., 2007), which could be observed in T1, T2 and T3 (Table 2). Fertilizers, mainly nitrogenous (like those used in this work) and ammoniacal ones, usually acidify the soil, which is beneficial in the preparation of the medium for the development of new seedlings (Seoánez-Calvo, Bellas-Velasco, Ladreda-Sureda, & Seoánez-Oliet, 2000), since at neutral pH most of the nutrients are available. After sowing, the most representative difference in pH values can be observed in T4 (B and C), followed by T0 (B); T2 and T3 fall within the established range and represent treatments with the lowest variation in pH (Table 2).
The compost OM percentage was 25.12 ± 0.42 (Table 1), which significantly improved soil OM content (T1, T2 and T3, Table 2). However, the increase in this parameter in T1, T2 and T3 is not proportional to the percentage increase in the compost-soil mixture; in fact, no significant difference between them is observed. This contrasts with the findings reported by Olivares-Campos et al. (2012), who obtained a significant average increase in compost trials.
The results obtained in this study could be related to optimum soil fertility, indicating that a 20 % proportion (w/w soil-compost) could be the most suitable to cover the requirements; a higher compost content could, in a given case, saturate it. In this case, Chang et al. (2007) demonstrated that applying compost to the soil over long periods can help to improve its OM content even after several plantings, and that some of the harvest benefits can be observed after the first planting. These authors conclude that an excess of OM can saturate the soil or substrate, thereby reducing its productivity. This agrees with what was observed in this work, since after the wilting of the maize and bean plants an increase in this parameter could be observed in the treatments with compost, especially in the case of T2 and T3.
TN content is usually used as an organic reserve index reference, both in the stratification of productive systems and in the balances of this element in soils (NOM-021-SEMARNAT-2000 [SEMARNAT, 2002]). Thus, the higher the TN content, the higher the OM reserve; however, as with other nutrients, a high content often causes fertility problems. Table 2 presents the results obtained for TN content. With respect to T0, before sowing, T1 had no difference in TN content, as opposed to T2 and T3, which showed an increase in comparison with T0, but no difference between them (Table 2).
The most significant increase in TN was presented by T4. T2, T3 and T4 improved their TN content, moving from a low to a high content level (NOM-021- SEMARNAT-2000 [SEMARNAT, 2002]). Casado-Vela et al. (2006) reported similar results, observing increased TN with increasing composted sewage sludge content. Compost treatments do not present homogeneous values, as mentioned previously. This behavior could be related to optimum soil fertility, indicating that a 20- 30 % proportion (w/w soil-compost) could be the most suitable to cover plant requirements, while a higher value does not result in soil improvements.
Table 3 presents the Bd, Pd, PS, M, FC and PWP values. It should be mentioned that these parameters did not show significant variation before and after sowing. Rodríguez-Fuentes y Rodríguez-Absi (2002) indicate that soil physical properties are difficult to modify in short periods. The main factors that affect these parameters are constant soil amendment use, tillage and natural or induced erosion.
Table 3 Physical parameter values of the treatments tested.
| Treatment | Bd* (g·cm-3) | Pd (g·cm-3) | PS (%) | M (%) | FC (mL·100 g-1) 1) | PWP (%) |
|---|---|---|---|---|---|---|
| T0 | 1.19 ± 0.173 | 2.00 | 34.12 ± 0.004 | 3.09 ± 0.005 | 4.50 ± 0.50 | 2.18 ± 0.243 |
| T1 | 1.05 ± 0.114 | 1.80 ± 0.015 | 35.16 ± 3.290 | 2.83 ± 0.047 | 5.00 ± 1.00 | 7.82 ± 1 |
| T2 | 0.91 ± 0.044 | 1.67 ± 0.004 | 47.80 ± 0.154 | 2.56 ± 0.068 | 6.00 ± 0.00 | 11.49 ± 3.853 |
| T3 | 0.82 ± 0.030 | 1.43 ± 0.001 | 41.05 ± 0.588 | 3.40 ± 0.076 | 6.00 ± 0.00 | 20.56 ± 3.85 |
| T4 | 1.28 ± 0.190 | 2.14 ± 0.022 | 34.29 ± 0.074 | 1.84 ± 0.094 | 5.00 ± 0.0002 | 2.43 ± 0.002 |
| T5 | 0.46 ± 0.021 | 0.91 | 50.73 ± 0.075 | 5.06 ± 0.102 | 33.50 ± 3.50 | 54.85 ± 28.18 |
*Bd = bulk density, Pd = particle density, M = moisture, FC = field capacity, PWP = permanent wilting point, T0 = negative control (100 % soil), T1 = soil plus 10 % compost, T2 = soil plus 20 % compost, T3 = soil plus 30 % compost, T4 = soil with chemical fertilizer and T5 = positive control (100 % compost).
Bd and Pd are important parameters, as they affect soil percolation, infiltration and aeration, and therefore root growth. Differences in these parameters and PS % were largely insignificant before and after planting for all treatments. As the compost content increased, the Bd and Pd decreased, obtaining an increase in PS %, with T2 having the highest PS % increase. Casado-Vela et al. (2006) reported different data since they obtained an increase in Pd and a decrease in Bd as the compost proportion increased. As a reference they indicated that the addition of 2 and 4 kg·m-2 of compost did not cause changes in these parameters. On the other hand, Olivares-Campos et al. (2012) observed that treatments with different proportions of compost and fertilizers did not present differences in Bd values.
As was observed, the treatments with compost showed a significant increase in OM content and a decrease in Bd with respect to the T0 control and T4 (Table 2). These results are in agreement with those reported by Rodríguez-Fuentes and Rodríguez-Absi (2002), who indicate that OM has an average Bd of 0.3 g·cm-3, so that as the amount of OM in the soil increases Bd usually decreases. Castellanos, Uvalle- Bueno, and Aguilar-Santelises (2000), Castillo, Quarín, and Iglesias (2002), Olivares-Campos et al. (2012), Pérez (2004) and Porta, López-Acevedo, and Roquero (1999) indicate that compost use directly impacts OM content, manifesting itself in a lower Bd, resulting in better soil fertility and porosity.
Considering the texture and clay content of the soil used in this work, it can be inferred that the soil has the following characteristics: regular to deficient infiltration capacity, medium to high FC and PWP, regular aeration, and a maximum PS % of 45 (Rodríguez-Fuentes & Rodríguez-Absi, 2002). In terms of useable water content, sandy and silty soils are the best, because the PWP and FC values are more distant from each other. However, high sand content can cause water losses, while very clayey soils tend to become saturated, causing the water to seek a way out, forming cracks and compacting the soil, thus preventing water from reaching the roots of plants.
The soil used has a low OM content and low M % (hygroscopic moisture, 2.51 ± 0.14). The results show that the treatments with compost had an increase in moisture content, with T3 being the one with the highest value (Table 3). In general, sandy soils have a FC of 5 to 16 %, clayey loam soils from 25 to 35 % and clayey ones from 30 to 70 % depending on the content and type of clay. The analyzed soil presented clayey loam texture, predominantly clayey with a Bd of 1,312 ± 0.005 g·cm-3 and a FC of 31.3 ± 0.13. Table 3 shows an increase in FC as the proportion of compost increases.
FC showed no significant differences between T1, T2 and T3, while PWP did show a significant increase, and T3 exhibited a difference compared to T1. The WHC of a soil varies according to the type of clay, OM content and structure (Rodríguez-Fuentes & Rodríguez-Absi, 2002). Generally, WHC values range from 1 to 2 mm in soils with very coarse sand, reaching up to 4 to 6.2 mm in soils with high clay content. T2 and T3 showed increased WHC compared to T0, T1 and T4; however, there was no significant difference between them (Figure 1).
Conclusions
Based on the analyzed parameters, the compost used, as established by the applicable legislation, can be used as a soil improver in urban green and reforested areas. The proportions of 10, 20 and 30 % of green waste compost did not modify soil pH, as compared to the fertilizer treatment that showed a significant decrease. The 20 and 30 % compost content levels acted as a cushion against pH changes after bean and maize sowing. The OM content presented an increase before and after maize and bean sowing, in all treatments with compost.
The fertilizer treatment showed the most significant difference in TN content; In turn, the treatments with compost did not present an increase. Compost application improved Pd and Bd by increasing the soil PS %, with T2 showing the largest increase. Adding compost helped to increase OM content, improving the water retention conditions and increasing the M % with 30 % compost (T3) and the FC and PWP as the compost dose increased, making the WHC increase with the addition of 20 and 30 %. Considering the formation characteristics of the analyzed soil, Bd, PS % and WHC are the most changeable soil properties over time. However, in this work it was verified that the addition of compost produced with green waste, even to a soil whose properties are not favorable for the development of plants, increased the soil OM content and helped to improve some of its characteristics.
