**2.1. Process of obtaining biosolids**

According to the legislative norms established by the Autonomous Community of Galiza (NW Spain), all sludge that originates from the purification process of urban wastewater must receive treatment prior to its use in order to sanitise and stabilise it, as well as reduce its volume. Thus, this waste product receives added value due to a chemical treatment process that sanitises and pasteurises sludge, which improves its use as a fertiliser. Both urban and industrial wastewater may be employed as an input. **Figure 1** shows a general outline of the entire purification process, which occurs in a timeframe of less than 1 h. Several chemical reagents are sequentially administered as a function of the type of treatment that is necessary to achieve sanitation. The system of reactors and ducts is closed and completely automatic, which minimises noise pollution and dust. Sludge is treated by an acidification process (with the addition of nitric, phosphoric and/or sulphuric acid) to sanitise and stabilise the organic residue, followed by its neutralisation with anhydrous ammonia. The product is submitted to a thermal process of drying and granulation. The entirety of the process is denominated as Verdiberia-Active Sludge Pasteurisation (Plateau-ASP-ActiSolids©), which includes an enhancement of fertilising capacity (via enrichment with organic N).


**Figure 1.** General process to obtain Verdiberia-ASP biosolid.

#### **2.2. Design of field assay and elemental analysis of biosolid and soils**

The field assay was conducted on a private agricultural farm in the municipality of Cospeito (Lugo province, Galiza, NW Spain; 43° 15' 13.15'' N, 7° 26' 31.76'' W). This municipality is characterised by an elevated intensity of agricultural-livestock use. **Table 1** lists the tempera‐


ture and rainfall conditions that correspond to the study period (June–October 2013), which are characterised by mild average temperatures and moderate precipitation [4].

**Table 1.** Average temperature (°C) and total rainfall (mL) during the study period (corn sawing–corn harvesting).

At the assay site, four parcels of 3 × 4 m2 were randomly and completely distributed for each of the following treatments:


The soil had an acidity level within the tolerable range for maize crops (pH of 5.5 in water and 10.7% saturation of Al, as an indicator of the cation-exchange capacity); thus, the parameters were not adjusted. To characterise the fertilisation treatments, the P levels of the soil were considered to be normal; however, the K levels were below the desired range. Based on these results and following the indications by [5], 190 kg ha−1 of N, 120 kg ha−1 of P2O5 and 260 kg ha −1 of K2O were defined as the base soil conditions for the maize crops in this study (**Table 2**).


**Table 2.** Initial main soil parameters (available K and P).

The site was prepared with a mouldboard and rotary plough. The fertilisers were completely incorporated in the soil in the study area according to common practice and uniformly distributed throughout the parcel 1-day prior to maize crop sowing as the final step in the process, which was performed on June 5, 2013. The Automat (Advanta) maize variety (*Zea mays* L.) was employed with a planting density of 70,000 plants ha−1. Potatoes had been cultivated the previous year before sowing, by this, the terrain was ploughed and the following phytosanitary treatments were applied as follows: 31.3% S-metolachlor herbicide + 18.7% terbuthylazine herbicide (4 L/ha; SIPCAM Inagra) and 48% liquid chlorpyrifos insectide (1 L/ ha).

Soil samples were collected before the maize sowing (control plots without application of fertiliser), 30 days after sowing and at the end of harvest on October 10, 2013.

The pH was measured in a 1:2.5 suspension of soil:water. The C and N contents were deter‐ mined using a Leco CNS 2000 Autoanalyser, and assimilable P was quantified by colorimetry following the Olsen method [6]. Several elements (Ca, Mg, Na, K and Al) were extracted with a 1 N solution of NH4Cl [7] and were quantified by spectrophotometry using atomic absorp‐ tion/emission techniques. Heavy metal soil content was measure through ICP before nitric acid digestion. The parameters that were measured for sludge are summarised in **Tables 3a** and **3b**.

To estimate production, cobs were harvested from ten central plants in each parcel. The fresh weight and dry weight were measured after dried in an oven at 60°C. The grains were subsequently separated to quantify the total production (kg ha−1) and the output (Harvest Index = dry weight grain/dry weight cob). In addition, the following weighted production indices were estimated as follows:

Sustainable Yield Index (SYI) [8]

ture and rainfall conditions that correspond to the study period (June–October 2013), which

are characterised by mild average temperatures and moderate precipitation [4].

of the following treatments:

**•** Control (CT): no application of fertiliser

276 Organic Fertilizers - From Basic Concepts to Applied Outcomes

**mg kg−1 cmol(+) kg−1**

**Table 2.** Initial main soil parameters (available K and P).

ha).

**2013 June July August September October** Temperature 16.1 18.2 18.5 16.4 12.9 Rainfall 52 34 36 68 137

**Table 1.** Average temperature (°C) and total rainfall (mL) during the study period (corn sawing–corn harvesting).

**•** Biosolid 1 (BS1): 3000 kg ha−1 VerdiberiaASP NP + 450 kg ha−1 of potassium sulphate.

**•** Biosolid 2 (BS2): 7000 kg ha−1 VerdiberiaASP NP + 450 kg ha−1 of potassium sulphate.

**•** Mineral fertiliser (MF): 1270 kg ha−1 de 15-15-15 + 140 kg ha−1 of potassium sulphate.

**pH K P Ca Mg Na K Al Sat Al** 5.52 179.66 41.05 3.50 0.70 0.11 0.46 0.57 10.74

The site was prepared with a mouldboard and rotary plough. The fertilisers were completely incorporated in the soil in the study area according to common practice and uniformly distributed throughout the parcel 1-day prior to maize crop sowing as the final step in the process, which was performed on June 5, 2013. The Automat (Advanta) maize variety (*Zea mays* L.) was employed with a planting density of 70,000 plants ha−1. Potatoes had been cultivated the previous year before sowing, by this, the terrain was ploughed and the following phytosanitary treatments were applied as follows: 31.3% S-metolachlor herbicide + 18.7% terbuthylazine herbicide (4 L/ha; SIPCAM Inagra) and 48% liquid chlorpyrifos insectide (1 L/

Soil samples were collected before the maize sowing (control plots without application of

fertiliser), 30 days after sowing and at the end of harvest on October 10, 2013.

The soil had an acidity level within the tolerable range for maize crops (pH of 5.5 in water and 10.7% saturation of Al, as an indicator of the cation-exchange capacity); thus, the parameters were not adjusted. To characterise the fertilisation treatments, the P levels of the soil were considered to be normal; however, the K levels were below the desired range. Based on these results and following the indications by [5], 190 kg ha−1 of N, 120 kg ha−1 of P2O5 and 260 kg ha −1 of K2O were defined as the base soil conditions for the maize crops in this study (**Table 2**).

At the assay site, four parcels of 3 × 4 m2 were randomly and completely distributed for each

$$SYI = \left(Y - \sigma\_{n-1}\right)Ym^{-1}, \text{ with } \sigma\_{n-1}$$

Y = fertilization treatment yield, σ = standard deviation, Ym = maximum yield (among all treatments) and in the same way the Relative Yield Index (RYI) was calculated, to establish a reference for maximum production with respect to the control:

$$RYI = \left(Y - Yc\right)Ym^{-1}, \text{ with }$$

Yc = Control treatment yield,

and the Agronomic Efficiency (AE) Index, to estimate the nitrogen use efficiency of the crop [9],

$$AE = \left(Y\_F - Y\_\bullet\right) Nap^{-1}\text{, with}$$

Nap = N applied in kg ha−1

#### **2.3. Statistical analysis**

The resulting means were compared by a two-factor analysis of variance (two-way ANOVA) with the goal of estimating the influence of the crop grow time period (30 days after sowing– harvest time, which varied according to the specific influence of the crop). A simple ANOVA was performed to determine the significance of the differences in the analysed parameters among treatments in terms of changes in soil characteristics and crop production. The normality of the data was verified (Kolmogorov–Smirnov test), and the homogeneity of variance was verified (Levene test). For cases in which the distribution of the variance was not homogeneous, the Games–Howell test was applied. Assuming homogenous variance, the least significant difference (LSD) method was employed. To compare with the control plot, a bilateral Dunnett's test was performed.

Bilateral Spearman's correlations were calculated for the different production indices, and the regressions between these previously uncorrelated indices and the dose of N and P that was administered by the distinct treatments were also established.

All statistical analyses were performed using SPSS [10].
