**3. Results**

## **3.1. Contribution of sludge to soil heavy metal content**

Considering the maximum concentration limits of heavy metals in the biosolid and its maximum final contribution to soils as a function of the utilised volume, neither surpassed the limits that were established by normative legislation for the heavy metals that were considered [11]. As listed in **Table 3a**, their levels fell below legal limits for all cases. For example, the Cr content and the Pb content were sixfold and 115-fold lower than the established kg ha−1 limits for the application of biosolids.

The biosolid obtained through the Active Sludge Pasteurisation processes shows a lower humidity percentage, for example [12] reported about 16.6% of dry matter, a low OM content [13] or more available P [14].


d.m. = dry matter, BS2 = Biosolid dose applied by 7000 kg ha−1.

**Table 3a.** Biosolid heavy metal content and legal limits (mg kg−1 d.m.).


Use of Pasteurised and N-Organic-Enriched Sewage Sludge (Biosolid) as Organic Fertiliser for Maize Crops: Grain Production and Soil Modification Evaluation http://dx.doi.org/10.5772/62813 279


**Table 3b.** Biosolid chemical characterization.

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

Considering the maximum concentration limits of heavy metals in the biosolid and its maximum final contribution to soils as a function of the utilised volume, neither surpassed the limits that were established by normative legislation for the heavy metals that were considered [11]. As listed in **Table 3a**, their levels fell below legal limits for all cases. For example, the Cr content and the Pb content were sixfold and 115-fold lower than the established kg ha−1 limits

The biosolid obtained through the Active Sludge Pasteurisation processes shows a lower humidity percentage, for example [12] reported about 16.6% of dry matter, a low OM content

Biosolid 69.5 168.3 20.5 450.2 27.95 3.0 2.23 Legal limit 1000 1000 750 2500 300 20 16 kg ha−1 (BS2) 0.4 1.1 0.13 2.9 0.18 0.02 0.01 Legal limit 3 12 15 30 3 0.15 0.10

**pH 7.98** EC (dS m−1) 48.8 Humidity (%) 9.56 OM (%d.m.) 24.16 C/N 1.39 N—total (%d.m.) 10.09 N—nitric (%d.m.) 1.02 N—ammonia (%d.m.) 2.49 N—urein (%d.m.) 0.24 N—organic (%d.m.) 6.34 P2O5—total (%d.m.) 8.72

**Cr Cu Pb Zn Ni Cd Hg**

administered by the distinct treatments were also established.

All statistical analyses were performed using SPSS [10].

278 Organic Fertilizers - From Basic Concepts to Applied Outcomes

**3.1. Contribution of sludge to soil heavy metal content**

**3. Results**

for the application of biosolids.

[13] or more available P [14].

d.m. = dry matter, BS2 = Biosolid dose applied by 7000 kg ha−1.

**Table 3a.** Biosolid heavy metal content and legal limits (mg kg−1 d.m.).

#### **3.2. Evaluation of sludge incorporation rate into soil**

The differences in the soil parameters were evaluated between the control parcels at the beginning of the experiment and the parcels that correspond to various treatments during the month of July, 30 days after the initial application of the different fertilisers (**Table 4**). These findings demonstrated a decrease in the pH levels of the treatments compared with CT and in the C/N and C/P ratios for the BS2 treatment, as well as a higher P availability in the same treatment, caused an increase in the relative concentrations of N and P; these elements are specifically provided by pasteurised and minerally enriched sludge. Given that sludge presents very low C/N, N/P and C/P ratios, as 62.8% of N is organic and 73.1% of P is present is its available form, and the soil pH showed a significant decrease after the initial application of sludge (considering the initial pH of the biosolid = 7.98), we can conclude that the fertilisers are rapidly incorporated in the soil. By other hand, during the month of June, a rainfall level of nearly 50 mLm2 and an average temperature of 16.1°C fostered the biological activity of the soil, which also favours the mineralisation process (**Table 1**).


Only significance differences are showed (different letter means statistical significance difference at α < 0.05). Pa = available P.

**Table 4.** Differences between fertiliser treatments and control 1 month after sawing time (Dunnett test). Mean and standard deviation values.

## **3.3. Analysis of temporal changes in soil nutrient content**

The independent comparison of the variables over the course of the evaluated time period in terms of their variation enable us to determine whether time serves a role in the evolution of soil characteristics (**Table 5**). After the first time period (30 days after fertilisation and sowing), a significant decrease in soil pH occurred in the BS2 treatment compared with CT and MF. At the end of the harvest, we also observed a decrease in pH for all treatments compared with CT. The average value of the decrease shifted from 0.9 to 0.5 pH units; however, this difference is not statistically significant (α = 0.25).

During the first period, only the BS2 treatment had a lower C/N ratio than the CT and MF treatments. In the second period, which corresponded to the end of the harvest, this ratio in BS2 was significantly lower than the CT, MF and BS1. This finding signifies that the relative quantity of N at the end of the experiment increased compared with the CT content for BS2 application (**Table 6**).

Differences in the N/P ratio were detected in the first period (BS2 < CT,MF), whereas BS2 treatment presented lower values for the second period compared with the CT and MF treatments, and BS1 lower than CT and MF.

Only the BS2 treatment demonstrated a lower C/P ratio over the course of the first period compared with the CT and MF. Over the course of the second period, BS1 and BS2 treatments presented lower values for this ratio compared with the CT and MF treatments, and the comparison of BS1 with BS2.

For the concentration of available P, the same behaviour was observed for the two time periods. Both BS1 and BS2 presented higher P values than that of the CT and MF, with differences between BS1 and BS2.


(J: 1 month after sawing time, O: harvest time).

Only soil characteristics with significance differences are showed. α = significance level.

**Table 5.** Differences between treatments for each period of soil sample.

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(J = sawing time, O = harvest time) OM, C, N, Alsat = Al saturation (%), Pa = available P (mg kg−1), Ka = available K, Mg, Na, Al (cmol+ kg−1).

Different letters show statistical differences (α < 0.05) between time periods for the same treatment.

**Table 6.** Mean and standard deviation values for soil parameters.

**3.3. Analysis of temporal changes in soil nutrient content**

is not statistically significant (α = 0.25).

280 Organic Fertilizers - From Basic Concepts to Applied Outcomes

treatments, and BS1 lower than CT and MF.

application (**Table 6**).

comparison of BS1 with BS2.

0.01 0.02

0.05 0.02 0.01

(J: 1 month after sawing time, O: harvest time).

BS1 < CT BS2 < MF BS2 < BS1

BS2 < CT BS2 < MF BS2 < BS1 0.04 0.03 0.03

0.01 0.01 0.01

Only soil characteristics with significance differences are showed. α = significance level.

**Table 5.** Differences between treatments for each period of soil sample.

between BS1 and BS2.

J BS2 < CT BS2 < MF

O MF < CT BS1 < CT BS2 < CT

The independent comparison of the variables over the course of the evaluated time period in terms of their variation enable us to determine whether time serves a role in the evolution of soil characteristics (**Table 5**). After the first time period (30 days after fertilisation and sowing), a significant decrease in soil pH occurred in the BS2 treatment compared with CT and MF. At the end of the harvest, we also observed a decrease in pH for all treatments compared with CT. The average value of the decrease shifted from 0.9 to 0.5 pH units; however, this difference

During the first period, only the BS2 treatment had a lower C/N ratio than the CT and MF treatments. In the second period, which corresponded to the end of the harvest, this ratio in BS2 was significantly lower than the CT, MF and BS1. This finding signifies that the relative quantity of N at the end of the experiment increased compared with the CT content for BS2

Differences in the N/P ratio were detected in the first period (BS2 < CT,MF), whereas BS2 treatment presented lower values for the second period compared with the CT and MF

Only the BS2 treatment demonstrated a lower C/P ratio over the course of the first period compared with the CT and MF. Over the course of the second period, BS1 and BS2 treatments presented lower values for this ratio compared with the CT and MF treatments, and the

For the concentration of available P, the same behaviour was observed for the two time periods. Both BS1 and BS2 presented higher P values than that of the CT and MF, with differences

**pH α C/N α N/P α C/P α Pa α**

0.01 0.03

0.01 0.01 0.01 0.01 0.01 BS2 < CT BS2 < MF

MF < CT BS1 < CT BS1 < MF BS2 < CT BS2 < MF BS2 < BS1 0.01 0.02

0.03 0.01 0.01 0.01 0.01 0.01 BS1 > CT BS1 > MF BS2 > CT BS2 > MF BS2 > BS1

BS1 > CT BS1 > MF BS2 > CT BS2 > MF BS2 > BS1 0.02 0.04 0.02 0.01 0.01

0.01 0.02 0.01 0.01 0.01

BS2 < CT BS2 < MF

BS1 < CT BS1 > MF BS2 < CT BS2 < MF BS2 < BS1 Comparing the two periods (July–October), significant differences are detected for the decreasing Ca and the available K content in soil for the CT and BS1 treatments, respectively. No other significant difference was detected. Based on these results, we propose that the applied fertiliser was sufficient for achieving adequate crop production, whereas the extraction of these elements and nutrients over the course of the study did not appear to serve an important role in the variation of their content in the soil.

#### **3.4. Total differences in the soil nutrient content as a function of cultivation time and applied treatments**

From a general perspective, differences between treatments were observed. The two-way ANOVA (time vs. treatment) only showed significant differences among the distinct treat‐ ments (α = 0.01); both time and the interaction of time with treatment type were not significant (α = 0.10, α = 0.90). These results confirm the results of the temporal evolution of soil charac‐ teristics as described in the previous section, which indicates that the factor of crop growth does not appear to serve a fundamental role in the modification of the soil parameters. However, the results indicate that the different treatments are responsible for the variations. Significant differences in pH values are detected, which significantly decreased over the course of the experiment relative to the control after the treatments with distinct doses of biosolid (not statistically significant between each treatment) and mineral fertilisation. However, differen‐ ces in the percentage of Al saturation, for which a decrease in pH does not represent a limitation of the availability of nutritional elements in the soil, were not observed.


**Table 7.** Soil parameters differences at harvest time (mean and standard deviations values). Only are showed significance differences (different letter mean significance difference at α < 0.05. Games–Howell test for pH and P). Pa = available P (mg kg−1).

The same tendency is detected in the relationships between nutrients and the C/N ratio, which has a significantly lower value for the BS2 treatment, in addition to the N/P and C/P ratios for BS1 and BS2. Anyway, the C/P and N/P rates are enough higher to promote a P soil accumu‐ lation [15, 16].

With regard to the available P content in the soil, we obtain a greater value in BS2 compared with the other treatments and a greater value in BS1 compared with the MF and CT. In a similar way [17] found more soil P after harvest time (**Table 7**). Therefore, we can assume that the application of biosolid produces a relative enrichment of N and P, which also indicates a potential eutrophication risk due to excess available P as it does not appear to be regulated by crop extraction. [13, 18] report the sewage sludge application as an available P fountain, and by other hand, the C/P and N/P ratios are mainly controlled by P supply [19]. Its content in the soil and the total quantity provided by the biosolid exceed the corresponding maximum limits that were established by legislative norms [11] (48 mg kg−1 for P). Based on these established limits, only one application per year of the product tested in this study is permissible. For P [20], recommended a maximum of 150 kg ha−1 in order to prevent eutrophication risk.

#### **3.5. Production, output and agronomic efficiency of crop**

Comparing the two periods (July–October), significant differences are detected for the decreasing Ca and the available K content in soil for the CT and BS1 treatments, respectively. No other significant difference was detected. Based on these results, we propose that the applied fertiliser was sufficient for achieving adequate crop production, whereas the extraction of these elements and nutrients over the course of the study did not appear to serve an

**3.4. Total differences in the soil nutrient content as a function of cultivation time and applied**

From a general perspective, differences between treatments were observed. The two-way ANOVA (time vs. treatment) only showed significant differences among the distinct treat‐ ments (α = 0.01); both time and the interaction of time with treatment type were not significant (α = 0.10, α = 0.90). These results confirm the results of the temporal evolution of soil charac‐ teristics as described in the previous section, which indicates that the factor of crop growth does not appear to serve a fundamental role in the modification of the soil parameters. However, the results indicate that the different treatments are responsible for the variations. Significant differences in pH values are detected, which significantly decreased over the course of the experiment relative to the control after the treatments with distinct doses of biosolid (not statistically significant between each treatment) and mineral fertilisation. However, differen‐ ces in the percentage of Al saturation, for which a decrease in pH does not represent a limitation

**pH C/N N/P C/P Pa**

1202.0 202.4 **a**

1018.4 150.6 **a**

649.3 76.9 **b**

451.7 120.3 **c**

37.7 5.7 **a**

42.2 6.6 **a**

63.2 6.6 **b**

108.9 19.4 **c**

89.4 13.6 **a**

77.7 11.4 **a**

49.8 6.5 **b**

39.0 11.6 **c**

**Table 7.** Soil parameters differences at harvest time (mean and standard deviations values). Only are showed significance differences (different letter mean significance difference at α < 0.05. Games–Howell test for pH and P). Pa =

The same tendency is detected in the relationships between nutrients and the C/N ratio, which has a significantly lower value for the BS2 treatment, in addition to the N/P and C/P ratios for

important role in the variation of their content in the soil.

282 Organic Fertilizers - From Basic Concepts to Applied Outcomes

of the availability of nutritional elements in the soil, were not observed.

13.4 0.4 **a**

13.1 0.5 **a**

13.1 0.5 **a**

11.7 0.4 **b**

**treatments**

CT 5.5

MF 5.1

BS1 5.0

BS2 4.8

available P (mg kg−1).

0.2 **a**

0.1 **b**

0.1 **b**

0.2 **b**

**Table 8** summarises the significant differences for the indices that are related with crop production: grain yield (kg ha−1), harvest index, RYI, SYI and AE and their correlations. Corn yield is significantly greater for the BS1 and BS2 treatments (similar for both) than for the CT and MF. In the same sense [21, 22], find largest corn production after sludge applying versus mineral fertilization.


**Table 8.** Differences found for production index and correlation values. Mean and standard deviation values.

For the RYI and SYI, differences follow a similar pattern to production levels. This finding may be attributed to the fact that the difference between production (BS vs. MF) and their low variability are the two factors with more influence in the yield index [yield coefficients of variation (%), CT = 34, MF = 23, BS1 = 16, BS2 = 16]. In the case of AE, the BS1 treatment was significantly greater than the corresponding BS2 treatment, which indicates that a direct relationship does not exist between the contribution of N and grain production. This tendency is confirmed when we analyse the regressions for the distinct production indices and different doses of N, which are not correlated. As observed in **Table 9**, the values of r2 are low, and a statistically significant relationship between the coefficients and the constants was not observed in any case. [23, 24] find that an excessive N fertilization results in low-use efficiency, without any yield benefits and long-term environmental consequences, soil acidification, Nleaching… [25] does not find any relationship between grain yield and N application rates (r = 0.26). Similar pattern was obtained to P addition and yield index, but, for example [26] find a positive relationship between P addition and corn yield response.


**Table 9.** Regression values for corn yield (Y) (kg ha−1) and harvest rate (HR) in relationship with N and P applied.
