It may be possible 1.  when you are using intensive cropping system with only cereals having high demand of N and adding no N fertilizer from outside for the last 2 years. 2. If you have more N leaching or denitrification problem.

Thanks Dr Tarafdar

The farms are permanent pasture based systems that apply average N fertilizer application rates of 300 kg/ha/year. Most of the farms use irrigation scheduling, so the soils are always kept at field capacity unless there has been a very heavy rainfall. Some of the farms are dry lands. According to my understanding, severe leaching or denitrification could occur under waterlogged conditions?

I stand corrected though.

Dear Phohlo,

Can your please tell us the pH of the soil?

Hi Portia,

Would you provide us with soil pH and textural class.

Is the grass harvested or used for grazing?

Dimpho & Manoj thanks for your inputs!

The soil pH (KCl) varies from 4.5-7.5 texture classes mostly sandy loam to loam. The pasture is grazed by dairy cows but sometimes grass is cut as silage. 

Hi Portia!:

Wild or induced fires may also bring about large losses of N and other nutrients.  If no significant  leaching, volatilization and/or extraction (for silage) has occurred in your farms, you should have your soil samples verified for possible errors!

Cheers!

Hi Portia,

In the second visit to these farms (two years later);

(1) Did you sample the same sampling site where you took samples before?

Within a large field, soil can be highly heterogeneous over few meters distance (spatial variation); that could lead to the variation you have observed in some farms.

(2) Did you notice any signs of soil erosion?  (Due to overgrazing? perhaps?). Few centimetres thickness of soil loss from the top soil can remove substantial amount of soil organic matter (and soil organic N with it).

-Susantha.

Were there any internal controls to verify repeatability of the assays?

Hi Portia,

thanks for your response.

1. Since your soil has low clay content, leaching could be a factor contributing to N losses.

2. Pastures (particularly ryegrass) mines a lot of N and if this grass is removed than N will be taken away with the crop.

Thank you very much for your responses they are all very thought provoking and insightful, they however leave me with even more questions.

@Miguel, fires are not a problem in this region as they rarely occur and I also considered the fact that the analysis could potentially be incorrect and had them re-analysed by a different lab, there were no significant differences.

@Susantha, we use a GPS generated sampling grid, so yes the areas we sampled are the sample as 2 years ago. No signs of erosion and farmers allocate their pastures according to feed demand and always make sure to try not to over graze. the management systems on these farms haven't changed since the last sampling, the fertilizer application rates are still relatively the same, how ever the climate has slightly changed especially in the winter. they are getting less rain than normal and some days are hotter. I forgot to mention that the re-sampling took place at varying times of the year than the previous sampling. Could seasonal variation have had an influence to that extent?

@Jack, there were no controls unfortunately!

@Dimpo, some of these farms have been operating for more than 20 years, we have seen previous analysis from before I stated sampling so as to see if the trend was there before. The previous analysis showed no trend, in fact there was fluctuation between years, but not a significant fluctuation hence i'm at a stand still.  

Portia and other colleagues, very interesting discussion . For sure we need georeferenced sampling appraisal for such studies to concisely pinpoint the changes any nutrient taking place spatially or temporally , unless the study site is so small.  I  hope , then , you would notice simultaneous reduction in organic carbon content  of the soil as well  . Loss of N through  leaching  , denitrification, over-grazing , erosion ( Do not know about the physiography of the site )  etc to account possible ways of total n reduction in such a short time , despite such heavy dose of annual N-application . 

Total nitrogen decreasing with in two years cropping to the tune of 50 percent is an interesting point for which very little reference is available at that time when total C is increasing.

kindly inform: 1. what crops were taken in that two years span ?. 2. was it a virgin soil brought to cultivation, if not previous cropping systems. 3. Was there any addition of organic materials, residue etc ?

Generally it can not so happen. Please indicate your initial C:N ratio, total N and C contents .

after receiving these information we proceed further.

Compared to other nutrient element,for  example phosphorus, nitrogen is a very mobile element in the soil, and that is why its loss in different forms is high if it  is not taken up by plants. Nitrogen in the soil may exist both as mineral organic form. As I mentioned above the life of inorganic form is not long in the soil, particularly in the humid tropical  soils. The decomposition of organic nitrogen in the labile form is also high. Optimizing the N-use efficiency of crops is very important in order to reduce the loss of N in various forms. Increasing efficiency of N not only improve farm income and profitability but also protecting the environment. By the way, according to Raun (2002) the global nitrogen recovery efficiency is about 33%, the rest 67% is unaccounted for, which is about 19 billion US Dollar.

Therefore, integration of organic amendments as integrated soil fertility and plant nutrient management approach can enhance and sustain agricultural productivity and reduce greenhouse gas emissions emissions through improved soil health.

What kind of means nitrogen? Total? NH4  or  NO3 ? N-NO 3 is easily leached by rain or irrigation. Moreover, the bacterial activity and microorganisms increased?

Portion talked about total N decrease to the tune of 50 percent in two years, and some increase in organic carbon which is practically impossible. The text books including Brady's, Russel's, Tiddler and Nelson or earlier Black's book or research references doesn't reveals this trends. If in two years, Soil gets depleted with 50% total N then soil prodtivity as such is at stake. Soil N as such does not depletes with out decreasing organic matter. Yes, it's true that soil mineral nitrogen may get depleted due to Cropping. 

Under optimum soil moisture and temperature, 4 – 5% of soil organic N could be mineralized in a given year.  Assuming that rate of organic N mineralization over 2-year period (and assuming no immobilisation), it would be possible to observe a maximum 10% drop of original organic N through mineralization followed by subsequent losses by denitrification and/or leaching (under a worst case soil management scenario).

Therefore, a drop of 50% organic N over two-year period has to be associated with some way of mechanical loss of organic matter (e.g. erosion?), unless the observed variation is not due to sampling errors (e.g. spatial variation).  Animal activities (e.g. old manure deposits) are a huge source of variation in pastures.

Since you have two years data for a range of farms, try to relate the results to soil management practices during the previous two years and other soil and climatic parameters in these farms to see if you may be able to understand the cause for the drastic reduction only in some farms.

Portia and other colleagues, knowing soil N dynamics, 50% reduction of total N over two years does not necessarily depict any major problem but all depends on some factors. Soil texture, pH, source of fertilizer, crop type, cultural practices and the list goes on. N leaching can occur if the soil is sandy, over irrigation or soil inundation. Volatilization of NO3 can occur at high pH and high temperature or dry soil. The followings suggestions may be helpful:

At what depth was the soil samples taken? I will recommend (if not already done) taken soil samples at different soil depth. This will help to determine N movement with soil depth which may be due to leaching.  

I think it will be a good idea to conduct N balance assessment on those farms to determine output/input ratio. Input: the amount of N applied (fertilizer and organic matter), output is majorly amount taken up by plants (tissue analysis) over the period of interest.  Taken soil sample alone may not give the detail information of soil N. This is going to give a diagnosis of the major output and then the next step will be clear.

Sampling protocols followed two years back and now are important. As pointed out  by Dr.Mahapatra, cropping history and management details  are required for interpretation of data. Have the sampling and analysis of soil samples done  by the same scientist ?Dr.Jayasundara  has mentioned almost all pertinent points which may  help in interpretation of data.

Dear Portia,

Lossing of nitrogen in soil sample after 2 years has several reasons: soil texture, pH, source of fertilizer, crop type, cultural practices, bacteria in soil. If the field has rotated wet-dry time  during 2 years; nitrogen may be losed by gas, irrigation. Some plants in the present filed can uptake nitrogen more and more than the previous. Some bateria in soil can take nitrogen and located inside their cells; and unsuitable analyse methods can cause the decrease. 

The decrease in N levels in soil (high C/N) may be attributed to:

1) the type of plant residues in the soil (high C:N ratio, higher C and lower N)…resulting in a  lower N mineralization rate: It has been reported that plant residues with a high C:N ratio (e.g. >30) are likely to decompose slowly with initial net immobilization of N, whereas residues with a smaller C:N ratio (e.g. legumes) are likely to decompose more rapidly with a net mineralization of N occurring right from the beginning.

2)There were possible losses of N by:

a) denitrification (soil bacteria use nitrate for their respiration in the place of oxygen in the air. This process occurs most rapidly in warm, wet soils with an abundance of nitrate).

 b) ammonia volatilization (High soil temperature,  High soil pH (>7.0), Moist soil or heavy dew, Low cation exchange capacity soil and poorly buffered soils (low soil organic matter, low bicarbonate content, high sand content)

c) leaching of mineralized N (heavy rain ).

Thus, nitrogen losses is affected by several factors including Soil type, physical and chemical properties of the soil, environmental conditions, agricultural practices….  

Until and unless the surface is scraped, reduction of half of the soil total N in two years of cropping is not at all possible. Dr. Subba Rao has talked about soil sampling protocols to be noted. In no way, we can make 50 percent of the soil total N in labile form, subject to various losses from the soil system, when the soil also shows some increase in organic carbon content.

Thank you all for the inputs, please see on the attachment a full soil analysis report for one of the farms. Maybe you can pick up something which I have missed.

Regards,

Portia

Ms.Portia,I have gone though your tabulated data in respect of several parameters. I feel that the total N is  much underestimated in 2016 analysis.So the total N analysis in 2016 samples may be  repeated again.In case of other parameters also the much deviating samples may be analysed  again,irrespective of year. In some cases ,both years samples need to be analysed simultaneouly in the same batch.

Dr Rao

Thank your for your prompt response, the samples have been re-analysed by a different lab and the differences were insignificant. My suspicion is on the initial analysis in 2014, there is no way of verifying those results as the samples have been thrown away already.

Hi Portia,

We have found that there is a linear relationship between total C and total N. This relationship is observed in 2014 data but not in 2016 data. See attached.

So I agree with Dr Rao, there must something wrong with 2016 N data.

If I may ask, how did you analyze for total N?

 Hi Dimpho

The lab we used to analyse total N uses elemental (total C and N) combustion analyser. This method is the same one they used in 2014. The lab which we used to compare the results with also uses the same analysis.

The procedure is fine. After working over more than three decades mainly on crops and Cropping Systems also N and organic matter as  major components and the review available, couldn't find loss of total N from the soil within two years of Cropping. The amount of labile N which have five fates cannot account for 50 percent of the soil total nitrogen. 

Also it shows carbon more or less equal after two years. I do strongly believe that the problem remains at the base point ie sampling. Would have thought that the soil developed huge solidity, dissolution of organic matter took place and also loss of total N from the soil. That was also not the case.

usually its differences in sampling causing such differences!

Judicious sampling and careful analysis should be considered. This may be useful.

There might be sampling error in your study resulted in 50 % depletion of total N in 2 years period.

There may be an error in the determination of nitrogen content in the soil two years ago, or now, more or two times are wrong

Dear Portia Phohlo,

Looking at your soil pH data your soil is most likely suffering from acid soil infertility.

Your first goal would be to get the pH to 5.7 to 6.3 to eliminate the toxicity of Aluminum and Manganese.  If your Magnesium test is less than 100 I suggest liming with Dolomitic lime and if over that use Calcitic limestone which is less expensive.

In addition to liming  I would suggest you get a more complete macro and micronutrient analysis.

Rapid losses of N from the soil can occur from 1) denitrification 2) leaching of Nitrate and 3) the loss through harvest of the nutrient not returned sufficiently to soil. Silage in particular can lead to rapid deterioration of the soil balance.

Your Nitrogen level can be increased by using legumes in your pastures and crop rotations. Sandy soils are very prone to nutrient losses and waterlogged soils. In your case you seem to be dealing with an acid sandy soil. In terms of nutrient and water retention Soil Carbon or organic matter levels are critical as well as balancing not only the pH but also the limiting macro and micro nutrients.

Recently I was looking at work in Australia which showed 80% soil Nitrogen loss when termites were eliminated from the soil. Termites can digest lignocellulose material high in Carbon by the symbiotic gut organisms which fix atmospheric Nitrogen. In order to prevent the loses of soil organic matter and Nitrogen the biological nitrogen fixation from legumes is critical and the return of plant residues and manures particular under composting is very useful in building the soil Carbon and Nitrogen levels. 

Best wishes for success in your research and studies. PRH

Dr.Hepperly,Good and useful interpretation of soil analysis data and suggestion, as expected.

Dear Portia, I concur with Dr Hepperly and the interpretation.  I think there might be a error in your determination to total N.  

Some more thoughts when optimizing the generation of fixed Soil Carbon the complex to mineral to clay and organic matter work together. The equilibrium level of improved soil organic matter is a function of Calcium plus clay. A working hypothesis is that as Calcium binds clay and organic matter together the organic matter which otherwise is open and vulnerable to to rapid microbial decay. As the complexed soil organic matters become less vulnerable to microbial act changing the equation of soil degradation where the primary production cannot keep pace with microbial degradation to one where the net photosynthesis out strips the net microbial degradation leading to soil building rather than soil mining is turned on its head with net photosynthesis lower in relation to reduced microbial decay.

The mineral clay mineral is the fine material which can wash easily and accumulate in subsoil layers. Luckily the World is awash with liming material and locally silt and clay can be used with the composting process to build permanently improved soils  a bioengineering of excellence in terms of soil organic matter.

Because soil organic matter leads to improved aeration and water retention and recycling it is factor which needs to targeted a useful side effect is its ability to reduce and mitigate the effects of greenhouse gas enrichment of the atmosphere and the difficulty of extreme climate events which are increasing under global warming models and scenarios.

The engagement of soil sequestration needs to come to policy table and be taught more conclusively in relation to our food, energy, environment and health future.  

Dr.Hepperly, the carbon sequestration through the clay-organic  complexes involving Ca ions  is of great iimportance in carbon stability  in soil and soil protection againest degradation. As has  been highlighted it has climate change mitigation potential.Stabilizing carbon in composting process deserves attention of all concerned. I appreciate  all the points mentioned and highlighted by Dr.Hepperly.

Dear Portia, there might have been considerable variation in soil sampling and analysis of N in the soil samples at two different times. Regardless of ecosystem and soil type, 50% depletion of total soil N in two years is inexplicable.

Dear Portia, I wish to further inform that the amount of N mineralized during one year period commonly ranges from 6 to 8% of the total N present in the soils of tropical Asia. You may go through the following publication for your reference: Kundu DK and Ladha JK (1999) Nutrient Cycling in Agroecosystems 53:19-33.

lots of words for a phenomenon which is moste likely caused by sampling errors, dear colleagues, as Suhanta mentioned at first. I have seen enough of this examples and its simply often that the colleagues even dont like to admit that there might be simple (sorry, often stupid) errors!. Especially when it comes to C, just calculate the deviations in masses and imagine how many lorries would have to drive up and down the road to let such bulk loads disappear! :-)

It can also depend on the crop. some of the plants have very high N-demean. Due to high N-uptake by the plant total soil N can be decreased. Also weather conditions can affect N content in the soil - with the high precipitations N-leaching is possible.