3.1. Grain Yield, N Content and NAE
The response of the barley crop to the application of fertilization during the three-year study period is shown in
Figure 2. The average barley grain yield in soils with fertilizer was higher than that of the unfertilized control soil (T0). The mineral fertilizer treatments (T1 and T2) resulted in higher yields than their organic counterparts (T3–T5), but no statistically significant differences were found. Mean production values were used in order to average out the effect of the environmental variations.
Average barley grain yields were 1150 kg·ha
−1 for the unfertilized control (T0), varied between 1650 and 1800 kg·ha
−1 for the DPM treatments (T3–T5) and reached 2450 kg·ha
−1 for the mineral fertilizer treatments (T1 and T2). In these latter cases, production was increased by 124% as compared to the control, while organic fertilizers led to an increase ranging from 51% (for T5) to 65% (for T3). This is in line with the findings of other studies [
14,
32], in which mineral fertilization also had a greater impact on crop yield and N content.
It is worth noting that the same average barley grain yield was obtained for T1 and T2 treatments. Thus, the application of the mineral fertilizer according to the needs of the crop (as is the case for T2, in agreement with López Bellido et al. [
24]), would involve significant savings (18 kg N·ha
−1) in comparison to the traditional mineral fertilization approach used in this region (T1).
The application of DPM as a fertilizer did not result in yields comparable to those usually attained with mineral fertilization (in 2010 and 2011), but differences were not statistically significant. Further, the increase in its application rate did not lead to a positive response in terms of crop yield (see
Figure 2 and
Table 2). The highest crop yields were attained for the lowest rate (9.5 t·ha
−1), suggesting that for higher application rates the potential benefits of DPM as a fertilizer would be affected by its toxicity, and by the decomposition rate, quantity of N immobilization and timing of nutrient release [
33,
34].
Several authors have reported that the sole application of organic fertilizers cannot result in the same performance levels as those attained with mineral fertilizers [
14,
32,
35], and that nitrification inhibitors improve ammonium supply in the soil, increasing crop performance [
36]. Hence, the larger increase in yield for the subplots treated with the mineral fertilizers than in those treated with DPM may be related to higher N availability in the early stages of crop development, even before the strong spring growth starts, due to the impact this has on the final crop production.
Regarding the N content in grain (
Table 2), it can be observed that the maximum values were associated with the mineral fertilizer (T1 and T2), in the 1.73% to 2.03% range, while the lowest values were found for the control soil (T0) (1.32% and 1.37%). The values for the organic fertilizer treatments ranged from 1.29% to 1.54%. Villar and Guillaumes [
35] also observed higher N content in wheat grain with mineral fertilization as compared to the application of pig slurry. Riley [
37] reported that the N content in grain increased with the mineral fertilizer application rate, from 1.68% to 2.17%, for application rates ranging from 0 to 120 kg N·ha
−1, respectively. These values are similar to those obtained in this study for T1 and T2 treatments. It is also worth mentioning that all values were below the maximum concentration limit (2.16% in barley) that would result in problems for the malting process [
38].
It should be noted that while the lowest rate of DPM (9.5 t·ha
−1) led to higher production yields than for the other two organic fertilization treatments, the N content in grain was lower. Higher crop yields entail higher N requirements and, if the availability of N in the form of nitrates is limited, it may result in a lower N content in grain [
38].
The NAE values (
Table 2), in a similar fashion to the other analyzed parameters, were affected by the type of fertilization, with values up to 18 and 20 kg grain·kg
−1 N for mineral fertilization (T1 and T2, respectively). As regards the treatments with organic fertilization, T3 obtained the highest values (up to 14 kg grain·kg
−1 N). In fact, in the last year of the experiment, the climatic conditions (with very low precipitation) led to a higher NAE value for T3 than for the mineral fertilizers. Delogu et al. [
30] determined that barley and wheat have similar responses to nitrogen fertilization, so NAE data for wheat can be used for comparison purposes (given that the data published on barley crops is scarcer). The highest values reported in the literature would range from 20 to 30 kg wheat grain·kg
−1 of applied N, in optimum cultivation conditions [
39,
40]. Delogu et al. [
30] obtained a value of 9 kg grain·kg
−1 N, while Duan et al. [
31] found NAE values in the 9 to 29 kg grain·kg
−1 N range, attaining the maximum values with chemical NPK combinations with manure. Villar and Guillaumes [
35] obtained values between −4.0 and 5.9 kg grain·kg
−1 N for pig slurry treatments without top-dress fertilizer, and values ranging from 13.9 to 16.9 kg grain·kg
−1 N when ENTEC
® mineral fertilizer was used. In this study, intervals similar to those reported by Villar and Guillaumes [
35] were obtained, with NAE values ranging from 1 to 14 kg grain·kg
−1 N for the treatments with DPM (T3- T5) and between 11 and 20 kg grain·kg
−1 N for the plots treated with ENTEC
® (T1 and T2). Nonetheless, it should be noted that these studies are not strictly comparable, as pig slurry features a lower C:N ratio and a higher ammonium content than manure.
3.4. Organic Matter, Nitrogen, Nitrates and Ammonium Contents in the Soil
The organic matter content (
Figure 3a) and total N content (
Figure 3b) at different soil depths showed similar trends, with higher values in the surface layer than in the deepest layer [
32] and in soils treated with DPM than in those to which mineral fertilization was added (or than in the control soil). Similar results have been reported in the literature [
32,
43,
45].
In the surface soil layer (S15), SOM content was in the 1.42%–1.65% range, while at a greater depth (S30) it was between 1.01% and 1.22%, mainly due to the minimum tillage system used, which did not homogenize the soil profile. Significantly different values were obtained in terms of the SOM content between the control soil and T5 organic fertilization treatment in S15, and as compared to T4 and T5 treatments in S30.
Organic matter showed a positive response to the increase in the application rate of DPM from 9.5 to 19.0 t·ha
−1, increasing by 3.5%, 8.5% and 14.4% for T3, T4 and T5, respectively, as compared to the values obtained in control soil in S15. This was an expected result, since organic fertilizers with high C:N (16 for the one studied herein) and low inorganic N content are known to be useful to increase organic matter content. The application of organic wastes at similar rates to those used herein has been reported to increase [
32,
43,
45,
46] or prevent the decrease of SOM content [
47], as is the case for T3. It should also be taken into consideration that the organic matter applied to calcareous soils is more stable, which also contributes to its increase [
43].
Regarding the total N content (
Figure 3b), no significant differences were found between treatments. The N content in S15 was in the 0.09–0.11% range, higher than in S30 (0.07–0.08%). In a similar fashion to the SOM, total N content was higher in the soils treated with DPM (T4 and T5) in S15. This nitrogen would not be available to the plant, being mostly organic N, leading to the low barley grain yields obtained in these subplots (
Figure 2). The lowest total N content corresponded to the subplots with organic fertilizer (T3), mineral fertilizer (T1 and T2) and the control (T0).
The SOM and the total N in the agricultural soil increased for all the fertilization treatments as compared to the control, obtaining the highest values when DPM was applied at rates higher than 14.2 t·ha
−1·year
−1 (133 kg N·ha
−1·year
−1). In the DPM treatments, the contribution of organic matter would be exogenous, whereas in the mineral fertilization ones the increase in SOM would only be due to crop residues such as roots [
32,
48], which would account for the slight increases (in SOM and total nitrogen) in S30 for T1 and T2.
At this point, it should be clarified that, whereas there would be a clear relationship between SOM and total N in the soil, such relationship would not be observed between SOM and mineral N (nitrate and ammonium) [
32]. Thus, there would be no relationship either between the mineral N and the total N contents, in line with the results discussed below.
The NO
3−-N content in the soil (
Figure 3c) did not significantly differ as a function of the type of fertilization treatment, neither in the surface layer nor in the deepest one. The average values ranged from 3.13 to 5.51 mg·kg
−1 in S15 and from 2.53 to 6.59 mg·kg
−1 in S30, and were always lower than 10 mg·kg
−1. The low nitrate values can be deemed normal, given that the sampling was conducted after the harvest, when the crop had already absorbed the maximum amounts of NO
3−-N.
Residual NO
3−-N contents were higher in the fertilized subplots (ranging from 20% to 77%) than in the control subplot. The highest soil nitrate enrichment did not correspond to the highest N application rates (T1 and T5 for mineral and organic fertilization, respectively), in contrast to other studies in which the maximum leaching nitrate losses were generated when the highest amounts of N were applied [
49,
50]. This will be discussed later on.
The average amounts of NH
4+-N (
Figure 3d) were significantly different for the surface layer in the subplots to which T4 treatment was applied. Further, the ammonium content values were higher in S15 (13.3–27.9 mg·kg
−1) than in S30 (11.5–21.2 mg·kg
−1) for all treatments, with a mean value lower than 20 mg·kg
−1 for all treatments except for T4. The mean ammonium content values were higher in the soils to which treatments were applied than in the control. The increase in the content of NH
4+–N in the subplots treated with DPM as compared to the control soil ranged from 19% to 109%, whereas the two mineral fertilizer treatments increased the NH
4+-N content in the soil surface by similar values (34% and 36%). Among the soils treated with DPM, the nitrate and ammonium values for T4 (14.2 t·ha
−1·year
−1) were higher than for T3 (9.5 t·ha
−1·year
−1) and T5 (19 t·ha
−1·year
−1).
In contrast to the nitrate anion, which features a high leaching capacity and high mobility through the soil profile [
15,
18], the ammonium cation has a greater capacity of retention in the soil [
51], which explains why its values were higher in the surface layer than in deeper layers.
The final content of nitrate and ammonium in soils was relatively low at the end of the crop harvest [
11]. The obtained values for NO
3-N content in the soil were similar to the results obtained by other authors upon application of pig slurry [
35,
50], sludge waste materials subjected to different stabilization treatments [
45], and mineral fertilization treatments with ENTEC
® [
35], with values generally lower than 10 mg·kg
−1 and no differences between mineral and organic treatments. Nitrate values below 20 mg·kg
−1 would be classified as ‘low’, according to Zhu et al. [
52]. The values for the NH
4+-N content would, in turn, be expected to be lower than 10 mg·kg
−1 and lower than the nitrate content [
10,
45,
50,
53]. However, the transformation process of ammonium into nitrate is strongly correlated with the pH [
54,
55,
56] and, in soils with pH≃8.0—as is the case in this experiment—a strong reduction in the nitrification process is known to occur [
56]. This would explain why they are higher than the nitrate content, being favored by ammonia volatilization, which would contribute to this decrease in inorganic N [
57].
It is also noteworthy that the highest crop yield values (
Figure 2) corresponded to those treatments (T1, T2 and T3) with higher nitrate amounts in S30 than in S15. This is an indication that those treatments would have been able to supply higher amounts of plant-available N (nitrates) than the other two (T4 and T5), which—after not being absorbed by the crop—would have been leached to deeper soil layers.
In relation to the mineral fertilization treatments (T1, with 108 kg N·ha
−1, and T2, with 90 kg N·ha
−1), the amounts of ammonium in the agricultural soil increased as the mineral fertilizer application rate was increased, whereas the nitrate content was higher in T2 than in T1. Soil texture is a relevant factor that affects the efficiency of nitrification inhibitors, and the inhibition effect of DMPP varies depending both on the type of soil and the material applied, increasing its efficiency with sand content. Fangueiro et al. [
11] determined that the nitrification inhibitory effect would last from 8 to 40 days, in such a way that its effect would be most favored in sandy soils—as is the case for the soils under study—given that the inhibitor is less likely to be retained in the soil. Nonetheless, this would have had a similar effect in both cases, so there must be a process that favors the nitrification of ammonia to nitrate in T2 as compared to T1. A reasonable explanation would be that the application of K (to meet crop needs in T2 treatment, and which was not supplied in T1) would favor the availability of NH
4+-N [
58] and, in turn, its nitrification.
The results in terms of production, nitrate and ammonium contents for the treatments with DPM suggest that some sort of inhibition of the organic N mineralization would have taken place, which would be directly proportional to the amount of organic waste material applied, and which would disappear over time (as in T4 treatment). The mineralization of the organic N provided by DPM seems to be higher for T3 treatment, while in T4 and T5 treatments it may be slower, resulting in insufficient amounts of nutrients for the crop, and, consequently, in lower crop yields.
The crop can uptake the N supplied by the mineral fertilizers during the year in which they were applied. Conversely, for the organic fertilizers at applications rates higher than 9.5 t·ha−1·year−1, part of the mineralized N may be immobilized by the microorganisms after its incorporation to the soil. It would be from the third year onwards when the mineralization processes would cause an increment vs. the immobilization processes (i.e., the immobilized N would be released in subsequent years).