Here is my translation of the first article Kiril linked for you guys. Reading just the translations may help some folks understand it easier.
This perception is at odds with a century of soil
organic C data reported herein for the Morrow Plots, the world’s
oldest experimental site under continuous corn (Zea mays L.).
After 40 to 50 yr of synthetic fertilization that exceeded grain N
removal by 60 to 190%, a net decline occurred in soil C despite
increasingly massive residue C incorporation, the decline being
more extensive for a corn–soybean (Glycine max L. Merr.) or
corn–oats (Avena sativa L.)–hay rotation than for continuous
corn and of greater intensity for the profile (0–46 cm) than the
Traditionally people believed that if you add excessive N fertilizers, the plant would grow more, adding more soil organic matter to the soil. However, on a test site that has had half to almost 4 times more N atoms applied (as fertilizer) than removed (as crops) for 40 years we find that instead of increasing SOM, it has declined. Specifically the SOM found up to 18 inches down in the soil.
These findings implicate fertilizer N in promoting the decomposition of crop residues and soil organic matter and are consistent with data from numerous cropping experiments involving synthetic N fertilization in the USA Corn Belt and elsewhere, although not with the interpretation usually provided.
Thus it appears that synthetic N promotes the decomposition of organic matter more than the buildup of it. The evidence for this is everywhere, including several "peer reviewed articles" even though in the past, no one has focused on this specific subject, or has "spoon fed" the information to the world.
There are important implications for soil C sequestration
because the yield-based input of fertilizer N has commonly
exceeded grain N removal for corn production on fertile soils
since the 1960s.
This is important to know/study because across the nation, especially with corn, we have been fertilizing in a way that reduces SOM for the last 50 years.
To mitigate the ongoing consequences of soil deterioration, atmospheric CO2 enrichment, and NO3– pollution of ground and surface waters, N fertilization should be managed by site-specific assessment of soil N availability.
To stop the problem, we should be fertilizing our corn fields based on how much N they need on a site to site basis. This will help reduce Nitrogen pollution, keep soil more fertile, and keep as much carbon sequestered in the soil as possible.
Current fertilizer N management practices, if combined with corn
stover removal for bioenergy production, exacerbate soil C loss.
Growing corn like we have, and now removing even more of the plant (the leaves and stock) for bio-fuel production, will increase the rate at which we lower our SOM levels.
With the introduction of chemical-based N management, a new
strategy became available for increasing not only grain yield, but
also the input of crop residues, which was assumed to be of value for
maintaining soil organic matter (SOM) (Lyon et al., 1952; Melsted,
1954; Tisdale and Nelson, 1956). Ample fertilizer N was believed to
promote humus formation by narrowing the C/N ratio of carbonaceous
residues and by providing a major elemental constituent (Lee
and Bray, 1949; Millar and Turk, 1951; Melsted, 1954).
50 years ago we thought we could increase both grain yields and soil quality by fertilizing synthetically and adding crop residue back into the ground. We thought the synthetic N would help humus develop because it would balance the C:N ratio (with the crop residue) and also provide N, which is part of many humic compounds.
(Then a bunch of lingo describing the history of the testing plots, how they calculated how much Carbon was returned to the soil each year as crop residue, how they measured the current levels of SOM, etc. Basically information for the person who wants to make sure their calculations are credible.)
If the input of C and N promotes accumulation of SOM,
then treatment effects should be apparent within the Morrow
Plots, which vary in the amount and frequency of these inputs
but not in soil type, climatic conditions, or tillage. These effects
should be reflected by Table 1, which compares SOC concentrations
for surface (0–15 cm) and subsurface (15–30 and 30–46
cm) samples collected before and after five decades of continuous
cropping with or without repeated NPK fertilization.
If we were right 50 years ago, we should be able to easily tell it based on these test plots because the only things that were different between them were the fertilizer, amendment, and C inputs.
As opposed to the usual assumption, fertilization was of
little, if any, benefit for soil C sequestration (Table 1). Rather,
the only significant SOC changes detected were net losses,
and these tended to be more extensive for the subsurface than
the surface soil and more serious for the HNPK subplots
than for the others studied. Both findings are consistent with
evidence that the addition of N or P is more effective for
stimulating mineralization of SOC in subsurface horizons,
as compared with surface soil layers (e.g., Rovira and Vallejo,
1997; Soon and Arshad, 2002; Fierer et al., 2003; Mack et al.,
It turns out we were wrong. Adding crop residue and fertilizer actually decreased the amount of organic C in the soil. Especially for the soil 6 to 18 inches down.
(Then some more jargon how people usually didn't test the soil at those depths so they had to calculate and estimate how much SOM was in the soil at those depths.)
The results, summarized by Table 2 for the plow layer (0–15 cm)
and profile (0–46 cm), provide no convincing evidence of soil C
sequestration in fertilized subplots despite the fact that C inputs
have benefited from a considerable increase in corn populations
since 1955 (from 20,000 or 30,000 to 69,000 plants ha−1). On
the contrary, a decline usually occurred that was more intensive
for the profile as a whole than for the plow layer.
Based on our data and calculations, there was no substantial increase in SOM over 50 years. This data is even more telling when we consider that because of the fertilizer, the plants have grown more, and thus we have added more crop residue back into the soil. But still, no increase in SOM. In fact a decrease was usually found.
Th e negative profile C balance observed in Table 2 for
chemical-based N management is most reasonably interpreted
as a net loss of the residue C returned within the past 51
growing seasons accompanied by a substantial decline in the
native SOC with atmospheric CO2 enrichment. Th is fi nding
is of particular interest for continuous corn and the corn–soybean
rotation and is consistent with data reported in Table
4 of Odell et al. (1982, 1984), although not with continued
acceptance of the authors’ interpretations.
Add every pound of Carbon we put into the ground to how much Carbon was in the ground before the experiments, and we find that we have lost a significant amount of Carbon. This agrees with some published studies back in 1982 and 1984. But people didn't believe those studies.
Th e first decade of commercial
fertilization brought a minor increase in soil C for previously
unamended subplots, but this was followed by a decline despite
dramatic escalation in the return of above- and belowground
residues as corn populations were increased progressively to
69,000 plants ha−1 by 2003.
For the first ten years SOM increased on some specific plots. But even as we grew more corn in the same place, and thus added more plant residue to the soil, SOM levels declined after the first decade.
The loss of SOC was more serious with the HNPK than the
NPK treatment despite a similar residue C input as estimated in
Plots that had high amounts of fertilizer lost more SOM than plots that had regular amounts of fertilizer. Even though the Carbon input between the plots was the same.
Among the three HNPK subplots, the decline in SOC was
much more extensive for the two rotations than for continuous
corn, despite a lower frequency of N fertilization. Th is fi nding
demonstrates the value of a greater input of highly carbonaceous
corn residue for reducing microbial use of SOC and emphasizes
the importance of fertilizer N management if corn stover is to be
harvested for bioenergy production. Special attention is also warranted
for the corn–soybean rotation that now dominates the USA
Corn Belt, in that C accumulation from manuring before introduction
of the HNPK treatment had disappeared within the plow
layer by 2005 (Fig. 2), with substantially greater profile C depletion
than for the NPK subplot in this rotation (Table 2). Th is finding
is consistent with previous reports of SOC decline when soils are
managed for corn and soybean production (Varvel, 1994, 2006;
Peters et al., 1997; Pikul et al., 2001; Olson et al., 2005).
In plots that had extra fertilizer, SOM decline was even more apparent when crop rotation with another crop took place. This is important because most farmers currently rotate their crops. This practice may be depleting their SOM even faster. After some time, even adding Manure between the rotations doesn't help build SOM. These findings agree with publications made int 1994, 2006, 1197, 2001, and 2005.
The foregoing observations fully support the value of N
fertilization at either rate studied for increasing biomass production
but not for sequestering SOC. This disparity would be
expected if fertilizer N enhances the activities of heterotrophic
soil microorganisms in using C derived from crop residues or
SOM. Such an effect was recognized long before the modern
era of synthetic N fertilizers (e.g., Starkey, 1924; White, 1927;
Waksman and Tenney, 1928) and has been verified more recently
in several laboratory and fi eld investigations (Gusser,
1970; Tóth, 1977; Reinertsen et al., 1984; Janzen and Kucey,
1988; Green et al., 1995; Vigil and Sparks, 1995; Soon and
Arshad, 2002; Fierer et al., 2003; Conde et al., 2005).
The evidence demonstrates that adding N to the soil increases the amount of microbes in the soil, but doesn't add to soil carbon sequestering. The difference between these two numbers demonstrates that N fertilizer enhances soil microbe activity.
Especially in their ability to break down crop residue and soil organic matter. This has been understood clear back in the mid 1920's. And more recently in 1970, 1977, 1984, 1988, 1995, 2002, 2003, 2005)
In other words, if you add N the microbes get happy and reproduce. They break down both plant residue and soil organic matter even faster now.
(This is at the core of the salt debate)
If N fertilization can have a negative effect on soil C sequestration,
the same trend observed for the Morrow Plots should be
readily evident from data collected in field studies elsewhere. Such
evidence is common in the scientific literature but has seldom been
acknowledged, perhaps because N fertilizer practices have been
predicated largely on short-term economic gain rather than longterm
These findings are not popular because they are often interpreted to mean that farmers will make less money right now if they consider the information.
(I would also say it isn't popular information amongst the organic crowd, at least in this forum, because it means that Synthetic N is good for microbes. At least short term. The sad part is, organic fertility is centered on SOM, and this should be at the core of investigation/discussion.)
A half century of synthetic N fertilization has played a crucial
role in expanding worldwide grain production, but there has
been a hidden cost to the soil resource: a net loss of native SOC
and the residue C inputs. This cost has been exacerbated by the
widespread use of yield-based systems for fertilizer N management,
which are advocated for the sake of short-term economic
gain rather than long-term sustainability. Fertilization beyond
crop N requirements could be reduced substantially by a shift
from yield- to soil-based N management, ideally implemented on
a site-specifi c basis.
The quick dollar has promoted the 'grow for quantity' idea to expand rapidly worldwide. This idea calculates fertilization needs based on plant requirements. Basing fertilizing needs on soil requirements can help alleviate this problem without adversely affecting production numbers.