2. THE AVAILABILITY OF MANURE-N TO CROPS
2.2.2.2. The Cumulative Effects of Repeated Manure Applications
Long-term experiments show that although annual release of organic N is very small, cumulatively these small contributions could have a large impact on the total N availability in a long-term period (Berntsen et al.
2007; Mallory et al., 2010; Schrửder, 2005).
In the 2 years following application of 15N-labeled cattle slurry applied by Sứrensen (2004) to a sandy loam in replicated micro plots, N uptake in barley in the year after application was 2–4 kg N ha−1 greater on the slurry- treated plots compared with the mineral-N treatment, and N uptake with the cover crop was 3–5 kg N ha−1 greater on the slurry-treated plots. How- ever, none of these differences were statistically significant (P < 0.05). In the second year after application, there were also no significant differences in N uptake. Sứrensen (2004) concluded that while a significant proportion of NH4+–N in cattle slurry is immobilized shortly after application to soil, the fertilizer value of the slurry may be slightly less or equal to the TAN content when gaseous N losses are avoided due to mineralization of organic N in the slurry. The immobilized N is only slowly released and contributes little to the residual effects in the first years.
The residual effects of 4 years repeated cattle slurry applications on grassland were studied by Schrửder et al. (2007) who reported that dry matter (DM) yields and N offtake of cut grassland responded positively
(P < 0.05) to both current manure applications and applications in previ- ous years, whereas mineral fertilizer-N increased yields only in the year of application. They found NFRV of injected cattle slurry of 51–53% in the first year and they calculated an NFRV of approximately 70% after 7–10 yearly applications. Nitrogen offtake could be reasonably well predicted with a simple N model, adopting an annual RDR of the organic N in manure of 0.10–0.33 year−1 during the year of application and 0.10 year−1 in the fol- lowing years, albeit keeping RDR at 0.10 year−1 for subsequent years is not in accordance with other results such as those presented in Table 7.5 above.
In contrast, it took two to four decades of yearly applications to raise the NFRV of surface-applied FYM to a similar level from an initial value of 31%.
Cela et al. (2011) evaluated the residual effects of pig slurry applied for 6 years to continuous maize. The residual effects of six consecutive pig slurry applications on wheat-N uptake ranged from 3 to 4% of the total N input. The authors compared these uptakes with the 2–5% in the year after slurry application reported by Sứrensen (2004) and Schrửder et al. (2007) in northern Europe. Cela et al. (2011) found that mineralization of the organic N applied in pig slurry, at rates compliant with the Nitrates Directive, con- tributed with further residual N effects and would allow farmers to reduce N fertilization of a subsequent wheat crop by around 30 kg N ha−1. This large residual effect could be due to greater proportions of residual mineral- N remaining in soil under dry Mediterranean conditions.
Hence, the accumulated effect of repeated manure application should be accounted for in fertilizer planning (Schrửder et al., 2007). Bosshard et al. (2009) showed that the previous field management, including previous manuring history, has insignificant influence on the availability of N in a new manure application.
Generally, the residual effect is greatest for manures containing a large proportion of organic N. As the mineralization of N in soil takes place dur- ing most of the year, the residual effect is greatest in crops with a long grow- ing season (Sứrensen and Amato, 2002); vice versa, losses of N are greater in crops with short growing season.
2.2.3. Implications of Longer Term Mineralization for Manure-N Efficiency
Although long-term N fertilization affects soil organic N reserves, N mineralization potential, and crop response to applied N, little information is available on the influence of short-term N fertilizer management on soil organic N availability and crop response (Glendining et al., 1997). This is a
concern because it is a common practice for farmers to repeatedly apply manure to the same fields. To accurately assess the total plant availability of manure nutrients and to ensure balanced fertilization, it is necessary to account for the nutrients remaining in soil from previous manure applica- tions. The amount of residual N that will be released during following years will increase the risk of excess N application if not accounted for when deciding on N fertilizer application rates. The long-term, as well as short- term, mineralization is affected by different parameters such as temperature, moisture, type of livestock manure, timing (Beckwith et al., 2002) and min- eral-N fertilizer use. Thus, the long-term effects of organic fertilizers through the slow release of N have to be considered to enable optimization of fertil- izer use. Given the long manuring history of most agricultural systems, a re- evaluation of the fertilizer value of manure appears justified (Schrửder et al., 2005). The results also imply that the long-term consequences of reduced N application rates may be underestimated if manuring histories are insuf- ficiently taken into account. This was also the conclusion of Shepherd (1993).
A number of simple models have been described to estimate the resid- ual effects of manure-N (e.g. Klausner et al., 1994; Petersen and Sứrensen, 2008a; Schrửder et al., 2007). In Table 7.6 an example of model-estimated residual N effects under North European conditions is given. This model is based on mineralization rates of residual organic N that diminish over the years after application.
We conclude that:
• While residual N effects are relatively small, even in the year after appli- cation, the accumulated effect of repeated manure application can be significant and should be accounted for in fertilizer planning.
• The N availability over the long term will be affected by the multiple parameters of manure type and composition, environmental parameters (moisture and temperature), application technique and timing.
Table 7.6 Residual Nitrogen Effects of a Single and Repeated Manure Applications Given as Mineral Fertilizer Replacement Values (% of Annual Total N Application)
Manure Type
Repeated Manure Applications
1 year 2 years 10 years
Cattle slurry 3–5% 5–7% 9–14%
Pig slurry 2–3% 3–5% 6–8%
Solid manure 5–8% 7–13% 12–24%
Model calculation from Petersen and Sứrensen (2008a).
• In the years following the season of application, the recovery of N is greater for more recalcitrant compounds than for readily available com- ponents such as urine, due to the slower release of N that enables crop recovery in later seasons.
• Most authors reported only small percentages of N availability for suc- cessive years, most of them being around 2–3% of extra N available per year, reaching average values from 50 to 80% for the total N availability in a 6–10 year period.