Difficultly Extractable Fixed Ammonium in Some Soilsof China

Ninety-three soil samples and 19 sedimentary rock samples collected from 21 provinces of China wereanalyzed for their contents of fixed ammonium and total N by Kjeldahl-HF method. Results showed thatamount of difficultly extractable fixed ammonium (the fixed ammonium that is not determinable by Kjeldahlprocedures commonly used for soils) in soils ranged from 0 to 202 mg kg--1. It was generally more than50 mg kg--1 in soils in Chanaii and Thrpan districts, Xinjiang, accounting for 3.2%~36.8% with an averageof 13.9% of the total N. For some Orthents derived from purple shale and purple sandstone in Sichuan andHunan provinces and Chao soils derived from secondary loess in Henan Province and Ningxia AutonomousRegion it was generally around 30 mg kg--1 , accounting for 4%~7% of the total soil N, and for most of therest of soils studied, with the exception of some subsoils, no or trace difficultly extractable fixed ammoniumcould be detected. It was suggested that the difficultly extractable fixed ammonium was originated fromParent rock, and for slightly weathered soils derived from parent materials rich in this form of N the Kjeldahlmethod might give underestimation of total soil N.

Fixed Ammonium in Major Types of Paddy Soils in Hunan Province, China

The contents, affecting factors, seasonal changes and availability of fixed ammonium in major types ofpaddy soils derived from different parent materials in Hunan Province, China, were studied using the Silva-Bremner method by laboratory and pot experiments. Results showed that the content of fixed ammoniumin the plough horizons ranged from 88.3 mg kg-1 to 388.1 mg kg-1, with 273.2 ± 77.7 mg kg-1 on average,accounting for 11.2% of total soil N on average. Content of fixed ammonium decreased in the order of newlylacustrine clayey paddy soil ＞ alluvial sandy paddy soil ＞ purple clayey paddy soil ＞ newly alluvial sandypaddy soil ＞ yellow clayey paddy soil ＞ reddish-yellow clayey paddy soil ＞ granitic sandy paddy soil. Therewere four distribution patterns of fixed ammonium in the profiles to 1-m depth, i.e., increase with the depth,decrease with increasing depth, no distinct change with the depth, and abrupt increase or decrease in somehorizon. Percentage of fixed ammonium in total N increased with the depth in most of the soils. Fixationof NH4+ by soil was higher at 30 ℃ than at 20 ℃ and 40 ℃, and continuous submergence benefited thefixation of NH4+ in newly alluvial sandy paddy soil, purple clayey paddy soil and alluvial sandy paddy soil,while alternating wetting and drying contributed to the fixation of NH4+ in yellow clayey paddy soil mostly.Fixed ammonium content in the test paddy soils was significantly correlated with ＜ 0.01 mm clay content(P ＜ 0.05), but not with ＜ 0.001 mm clay content, total N, organic N and organic matter. Fixed ammoniumcontent varied with rice growth stages. Application of N fertilizer promoted fixation of NH4+ by soil, and Nuptake by rice plant promoted release of fixed ammonium from the soil. Recently fixed ammonium in paddysoil after N fertilizer application was nearly 100% available to rice plant, while native fixed ammonium wasonly partly available, varying with the soil type and rice type.

Fixed Ammonium Content and Maximum Capacity of Ammonium Fixation in Major Types of Tillage Soils in Hunan Province,China

In order to understand the status of fixed ammonium, fixed ammonium content, maximum capacity of ammonium fixation, and their influencing factors in major types of tillage soils of Hunan Province, China, were studied with sampling on fields, and laboratory incubation and determination. The main results are summarized as follows： （1） Content of fixed ammonium in the tested soils varies greatly with soil use pattern and the nature of parent material. For the paddy soils, it ranges from 135.4 ± 57.4 to 412.8±32,4 mg kg^-1, with 304.7±96.7 mg kg^-1 in average; while it ranges from 59.4 to 435.7 mg kg^-1, with 230.1 ± 89.2 mg kg^-1 in average for the upland soils. The soils developed from limnic material and slate had higher fixed ammonium content than the soils developed from granite. The percentage of fixed ammonium to total N in the upland soils is always higher than that in the paddy soils. It ranges from 6.1 ±3.6% to 16.6±4.6%, with 14.0% ±5.1% in average for the paddy soils and it amounted to 5.8±2.0% to 40.1 ± 17.8%, with 23.5 ± 14.2% in average for upland soils. （2） The maximum capacity of ammonium fixation has the same trend with the fixed ammonium content in the tested soils. For all the tested soils, the percentage of recently fixed ammonium to maximum capacity of ammonium fixation is always bellow 20% and it may be due to the fact that the soils have high fertility and high saturation of ammonium-fixing site. （3） The clay content and clay composition in the tested soils are the two important factors influencing their fixed ammonium content and maximum capacity of ammonium fixation. The results showed that hydrous mica is the main 2：1 type clay mineral in 〈 0.02 mm clay of the paddy soils, and its content in 0.02-0.002 mm clay is much higher than that in 〈 0.002 mm clay of the soils. The statistical analysis showed that both the fixed ammonium content and the maximum capacity of ammonium fixation of the paddy soils were positively correlated with the total 2：1 type clay mineral content and hydrous mica content in 〈 0.02 mm clay and 0,02-0.002 mm clay at the significant level of P0.01 or P0.05, respectively, but not correlated with the total 2：1 type clay mineral content and hydrous mica content in 〈0.002 mm clay significantly, It demonstrated that the ammonium-fixing matrix of the tested soils mainly exists in the 0.02-0.002 mm clay. The above-mentioned results showed that fixed ammonium is a major form of N in the tillage soils in Hunan Province, China. It would play an important role in N nutrition of crops, especially for upland crops, and deserve to be further researched.

Natural Nitrogen-15 Abundance of Ammonium Nitrogen and Fixed Ammonium in Soils

The present article deals with the natural nitrogen-15 abundance of ammonium nitrogen and fixed ammonium in different soils. Variations in the natural 15N abundance of ammonium nitrogen mineralized in soils under anaerobic incubation condition were related to soil pH. The δ 15N of mineralizable N in acid soils was lower but that in neutral and calcareous soils was higher compared with the δ 15N of total N in the soils. A variation tendency was also found in the δ 15N of amino-acid N in the hydrolysates of soils. The natural 15N abundance of fixed ammonium was higher than that of total N in most surface soils and other soil horizons, indicating that the increase of δ 15N in the soil horizons beneath subsurface horizon of some forest soils and acid paddy soils was related to the higher δ 15N value of fixed ammonium in the soil.

A Method for Determining Available Fixed Ammoniumin Soils

Dynamics of fixed NH in NH-treated soils incubated with glucose at 37±2℃ during the course ofincubation and factors affecting it were studied. Results showed that content of fixed NH in soil reached aminimun on day 7 after incubation and then increased gradually regardless of the amount of glucose addedand the kind of soil tested. However, the amount of fixed NH released from the soil at the given time variedwith both the amount of glucose added and the kind of soil examined. In cases glucose was added at a rateof 10.0g C/kg soil, the amount of fixed NH retained in soil after 7 days of incubation was almost identicalto that found by Neubauer test. Addition of K￣+ depressed the release of fixed NH significantly. Based onthe results obtained a method for determining the content of available fixed NH in soils was proposed andthe amount of N as available fixed NH in two soils measured by this method on an area profile-depth basiswas presented.

Does the Total Soil N Determined by Kjeldahl Method Include Fixed NH4^+?

Forty soils from top layer (0-20 cm) were sampled in different regions of China and Kjeldahl, HF-Kjeldahl and double treatment methods were used to determine total N, total N plus fixed ammonium, and total N and the residual fixed ammonium left in soil after determination of total N, respectively, to evaluate if Kjeldahl’s method could include the fixed N by soil minerals. The fixed N by soil minerals was measured by Silva-Bremner procedure to make comparison. Results showed that total N determined by Kjeldahl’s method averaged 1.622 g kg-1, while that by HF- Kjeldahl’s method 1.633 g kg-1, and that by double procedure 1.666 g kg-1. Obviously results obtained by the last two methods, particularly the double treatment method, were higher than Kjeldahl’s, showing that Kjeldahl’s method could not or not fully release N fixed by 2:1 minerals in soil, and therefore the determined results would not be the true “total N” for soils that contained large amount of the fixed N. The mineral fixedN averaged 166 mg kg-1, accounting for 10.1% of the total N while the residual fixed N amounted to 30.4 mg kg-1, equivalent to 1.9% of the total N or 18.3% of the total fixed N. The residual fixed N was correlated neither to organic matter nor to total N, but closely related to the total fixed N with a correlation coefficient of 0.598 (n=40), showing that the fixed N was the sole source of the residues. Soils having high residues of the fixed N were just those containing high fixed N, and soils containing high fixed N were just those containing high amount of 2:1 minerals. As a result, Kjeldahl’s method could not give a true value of the total N for such soils. However, for those containing small or little amount of 2:1 minerals, there was no significant difference in results measured by these methods.