Mineralization of organic matter and productivity of tifton 85 grass (Cynodon spp.) in soil incorporated with stabilized sludge from a vertical flow constructed wetland

Little is known regarding how to discard the sludge accumulated in vertical flow constructed wetlands (VF-CWs) and what the potential impacts could be. The objective of this paper was to evaluate the mineralization of organic matter (OM) in soil and productivity of tifton 85 grass (Cynodon spp.) after incorporating sludge collected at different depths from a VF-CW (used to treat septic tank sludge), to a tropical soil (Oxisol). Sludge samples were collected at depths of 0–5, 5–10 and 10–15 cm from a VF-CW that was used over a period of three years. The sludge collected at each depth was incorporated into the soil at a dose equivalent to 30 g m year of total nitrogen, and the experimental area was planted. During a period of 215 days, total and easily oxidizable carbon, total, ammonia, nitric and organic nitrogen in the residue-soil mixtures were analyzed. Based on the data obtained, the mineralization fractions (MF) were estimated for the specific monitoring period and annually considering first order and two-phase kinetic equations. The annual MF of the OM were higher than 96% and the sludge-amended soil resulted in an increase in grass yield. doi: 10.2166/washdev.2019.133 s://iwaponline.com/washdev/article-pdf/9/2/309/612715/washdev0090309.pdf Diogo André P. Silva Antonio T. Matos (corresponding author) Belo Horizonte, MG, Brazil E-mail: atmatos@desa.ufmg.br Mateus P. Matos Lavras, MG, Brazil


INTRODUCTION
There are several technologies for the treatment/disposal of sewage sludge; one such technique is a vertical flow constructed wetland (VF-CWs), also referred to as 'planted drying beds'. According to Suntti et al. () and Andrade et al. (), planted VF-CWs are a natural and decentralized system for the treatment of anaerobic or aerobic sludge, with low implantation costs, operational simplicity, low energy consumption and they are appropriate for diverse situations, in addition to not requiring the addition of chemicals to perform dewatering. According to Stefanakis & Tsihrintzis (), the sewage sludge is applied superficially to a porous and planted substrate, where over time it is dehydrated, resulting in the accumulation of a dark-colored 'biosolid', which is rich in organic matter. However, for the system to function properly it is necessary that the accumulated organic matter be removed from time to time so the VF-CW can be used continuously, without interruptions to its operation.
As the most attractive alternative, the accumulated sludge removed from a VF-CW can be incorporated into the soil, because this material improves the chemical, physical and biological properties of soils, promotes an increase in agricultural productivity and reduces the costs of soil recovery (Ferrer et al. ). Agriculture has long recognized the benefits of waste materials as a nutrient source and as an amendment to improve the physical and chemical properties of soils (Alvarenga et al. ).
The usual criteria for determining the annual application rate of sewage sludge in agricultural soils is based on the mineralization of this residue, mainly by evaluating the nitrogen availability (Doublet et al. ). The annual mineralization fraction of the sewage sludge therefore depends on the soil type into which it was incorporated, the treatment it underwent, and the rate at which it was applied (Parnaudeau et al. ).
The quantification of organic carbon and nitrogen mineralization can be obtained under field or climatecontrolled conditions in the laboratory, which is easier to install and operate, but often disregards the infinity of physical, chemical and biological processes that influence the mineralization of organic matter in field conditions. The dynamics of organic matter mineralization in soils have been commonly expressed by first-order chemical kinetic equations (Stanford & Smith ), as well as by the exponential two-phase model (Inobushi et al. ).
However, there is little knowledge regarding mineralization and the fertilization potential of the sludge accumulated in VF-CWs. The objective of this study was therefore to evaluate the mineralization of organic matter in sludge and the productivity of tifton 85 grass (Cynodon spp.) after incorporating sewage sludge collected at different depths from a VF-CW, to a tropical soil (Oxisol). The organic material under analysis was accumulated in a 15 cm layer after three years of disposal from a septic tank to the VF-CW which was planted with tifton 85 grass (Cynodon spp.). Organic material accumulated in the VF-CW (Figure 1(a)) was collected from the layers at 0-5, 5-10 and 10-15-cm (Figure 1(b)) at random spots throughout the bed and mixed to obtain a composite sample. The layers are represented by the acronyms: CWL 0-5, CWL 5-10 and CWL 10-15.

METHODS
To characterize the organic residues collected on the VF-CW and the soil to which it was applied, pH was   Table 1 presents the chemical and physical characterization of soil samples and organic residues accumulated at different depths in the VF-CW. Based on the results, it was verified that the soil to which the sludge was incorporated is clayey, which, from an agricultural point of view, has low available phosphorus, high available potassium, good Ca þ Mg, very low exchangeable Al 3þ , good pH and very low potential acidity (H þ Al). Therefore, chemically there appears to be no restrictions on the activity of microorganisms to decompose the organic material.
With regards to the organic residues collected in the VF-CW, the OOC and TOC contents were lower in the deeper collection layers. These results corroborate the studies of

)
. From the observed OOC/TOC ratio, it was expected that mineralization of organic material from the residues incorporated to the soil would be low due to the presence of a smaller quantity of labile C (C-carbohydrates) and a higher presence of stable recalcitrant compounds.
An experiment was conducted to quantify organic matter mineralization of the organic residues collected in the VF-CW, and its potential as a fertilizer considering biomass Table 1 | Chemical and physical characteristics of the soil (air dried fine earth) and the organic materials collected at the depths of 0-5 cm (CWL 0-5), 5-10 cm (CWL 5-10) and 10-15 cm (CWL 10-15) of the VF-CW

Soil
Organic residues accumulated in the VF-CW  To describe the mineralization of soil TOC, OOC and ON, the models used were the first order simple exponential model (Equation (1)) proposed by Stanford & Smith (), and the two-phase model (Equation (2) where M (min)cal and M 0 (min)cal are the accumulated mineralized mass calculated for a given degradation time per unit mass of the soil-residue mixture (dag kg -1 or mg kg -1 ); M (pot) is the potentially mineralizable mass per unit mass of the soil-residue mixture (dag kg -1 or mg kg -1 ); k is the mineralization coefficient (d -1 ); t is the monitoring time after incorporating the organic material into the soil (d); Although the accumulated mineralized masses were calculated using both Equations (1) and (2), the mineralization fraction of organic C and N was estimated using only the conceptual model (Equation (3)) and the parameters obtained in the two-phase model (Equation (4)): To evaluate the aerial biomass productivity of tifton 85 grass, the aerial portion of the plant was cut to 3 cm in relation to the soil surface to quantify the dry matter productivity 180 days after planting. The results were analyzed via analysis of variance and the means were compared using the Tukey test, adopting 5% significance.

RESULTS
The   Figure 3(i)), which was greater than that obtained at the depth of 5-10 cm (Figure 3(h)). The high mineralization coefficients (k) obtained in the first days after incorporating the organic residue into the soil can be justified by the OOC/ TOC ratio of the organic residues equal to 0.57 (Table 1). Matos et al. () reported an OOC/TOC ratio of 0.31 in    Lemainski & Silva () also reported that in a corn cultivation experiment, fertilization with sewage sludge was on average 21% more efficient than mineral fertilizer. In the present study there was an 11-fold increase in productivity of tifton 85 grass when fertilizing with VF-CW organic residue in relation to the zero dose of the control soil. In addition to improvements to the chemical conditions in the medium, it may be suspected that the improvement of physical properties may also have been a contributing factor to obtain greater productivity of the grass. It is believed that greater water retention was the factor that had the greatest influence on the results. According to Ferrer et al. (), the incorporation of sanitary sewage sludge improves the physical properties (structure and texture) of the soil, and can greatly influence plant growth.
The high productivity of tifton 85 grass fertilized with organic residues has been observed in several scientific articles. In a study by Matos et al. (), tifton 85 grass cultivated for four months showed high dry matter accumulation (1,500 g m -2 ), and a relatively high capacity to remove nutrients (nitrogen, phosphorus and potassium) and sodium from dairy wastewater applied as fertigation to crops. Fia et al.