Agriculture Reference
In-Depth Information
Assessment of Biogeochemical Fluxes under Different
Management Strategies Using SALUS
Nitrate Leaching Following Manure Application
Organic sources of N are often considered superior to inorganic fertilizers because
they decompose slower and promote better soil structure and overall soil quality.
However, there has been little field-based research to quantify nitrate leaching
when animal manure is applied as the primary source of nutrients in intensive crop
production systems. It is possible that when organic sources of N fertilizer are used,
nitrate leaching may be greater than when using inorganic N because organic N is
converted to inorganic N only slowly, so large quantities of organic N are needed to
provide enough N to rapidly growing crop plants during the relatively short time of
intense N uptake. More surplus N may then be mineralized and available for leach-
ing at the end of the growing season.
Basso and Ritchie (2005) quantified N leaching from KBS plots receiving
large quantities of either animal manure (18 ton ha −1 yr −1 ) or inorganic N fertil-
izer (120 kg N ha −1 yr −1 ) from January 1994 to December 1999 in a corn-alfalfa
rotation. The results were used to validate the ability of SALUS to simulate
nitrate leaching. Most of the water drainage occurred early in the season or
after harvest and was lower during the growing period of the crop. SALUS
provided a reasonable simulation of the amount of water drained and nitrate
leached for both manure and inorganic N fertilizer over the 6 years of the study
(Table 10.5). The manure plots leached 33% more N as nitrate (NO 3 ) than did
the plots treated with inorganic N, illustrating the trade-off between the organic
matter benefits of manure and a greater N loss to the environment (Millar and
Robertson 2015, Chapter 9 in this volume). Field studies and the validated
model results showed that leaching can be substantial if a high quantity of
manure is applied to soils in autumn (Basso and Ritchie 2005, Beckwith et al.
1998, Chambers et al. 2000).
Soil Carbon Changes in Cropped and Unmanaged Ecosystems
Soil tillage has contributed significantly to the increase in atmospheric CO 2 that has
occurred over the last two centuries (Wilson 1978). Historically, intensive tillage of
agricultural soils has led to substantial losses of soil C, ranging from 30 to 50% of
preconversion levels (Davidson and Ackerman 1993). These CO 2 losses are related
to soil fracturing and opening, which facilitates the movement of CO 2 out of—and
oxygen into—the soil (Reicosky 1997, Lal 2004), and especially to the destruction
of soil aggregates (e.g., Grandy and Robertson 2006; Paul et al. 2015, Chapter 5
in this volume), which exposes otherwise protected organic matter to microbial
attack. Although conventional moldboard plowing buries nearly all plant residue,
it leaves the soil in a rough, loose, and open condition, which maximizes CO 2 loss
and results in a consistent reduction in SOM. Reduced tillage results in more soil
C retention or sequestration, which reduces its atmospheric release (Cole 1996,
Paustian et al. 1998, Rasmussen et al. 1998).
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